docs(query-builder): add Query Builder Model topic

[teodosiah]

Closes #6054

Checklist:

• check topic's TOC/menu and paragraph headings
• Include TOC topic labels in topic content has a valuable update, it's new or considered as preview\ beta
• link to other topics using ../relative/path.md
• at the References section at the end of the topic add links to topics, samples, etc
• reference API documentation instead of adding a section with API

• use valid component names - [Data] Grid, IgxSelectComponent, <igx-combo>
• use spell checker tool (VS Code, Grammarly, Microsoft Editor)
• add inline code blocks for the names of classes / tags / properties
• add language descriptor for the code blocks
• check broken links (use browser add-on)
• check if sample is working and fully visible in the topic
• check if sample is working and fully visible in the StackBlitz
• check if code blocks match the code in StackBlitz demo

• follow SEO guidelines to fill topic's metadata (SEO guidelines)

• do not resolve requested changes (leave that to the reviewer)
• add pending-localization label when the review of the PR is done
• add a member from the localization team to translate it

[Otixa]

The whole conditionName: IgxStringFilteringOperand.instance().condition('endsWith').name reads a bit weird, to be honest, but it's technically correct and it illustrates where you can get the conditions and names.

[gedinakova]

Move this section before the SQL Example section

[gedinakova]

Suggested change

	The demo below demonstrates how the [`IgxQueryBuilderComponent`]({environment:angularApiUrl}/classes/igxquerybuildercomponent.html) expression tree could be used to requets data from an endpoint [Northwind WebAPI](https://data-northwind.indigo.design/swagger/index.html) and set it as [`IgxGridComponent`]({environment:angularApiUrl}/classes/igxgridcomponent.html) data source.
	The demo below demonstrates how the [`IgxQueryBuilderComponent`]({environment:angularApiUrl}/classes/igxquerybuildercomponent.html) expression tree could be used to request data from an endpoint [Northwind WebAPI](https://data-northwind.indigo.design/swagger/index.html) and set it as [`IgxGridComponent`]({environment:angularApiUrl}/classes/igxgridcomponent.html) data source.

[gedinakova]

Suggested change

	After subscribing to the request, the received data should be parsed to proper data type and set it as data source of the [`IgxGridComponent`]({environment:angularApiUrl}/classes/igxgridcomponent.html).
	After subscribing to the request, the received data should be parsed to a proper data type and set as the data source of the [`IgxGridComponent`]({environment:angularApiUrl}/classes/igxgridcomponent.html).

[ChronosSF]

While this sample can remain here to explain how the customer uses the api to make these requests, the "request sample" should be the main sample in the main topic, instead of the one simply listing the query as text.

[ChronosSF]

Basically as a showcase for the QB. The main topic doesn't have to provide any additional explanation to what the sample is about.

outdated

[Otixa]

Suggested change

	_description: Angular Query Builder provides easily serializable/deserializable JSON format model, making it easy to build SQL queries. Try it Now.
	_description: Angular Query Builder provides an easily serializable/deserializable JSON format model, making it easy to build SQL queries. Try it now.

[Otixa]

Suggested change

	Angular Query Builder provides serializable/deserializable JSON format model, making it easy to build SQL queries.
	Angular Query Builder provides an easily serializable/deserializable JSON format model, making it easy to build SQL queries.

[Otixa]

Suggested change

	title: Using Query Builder Model
	title: Using the Query Builder Model

[Otixa]

Suggested change

	# Using Query Builder Model
	# Using the Query Builder Model

[Otixa]

Suggested change

	Selecting the *IN / NOT-IN* operator in a `FilteringExpression` would create a subquery. After choosing entity and a column to return, it checks if the value in the specified column in the outer query matches or not any of the values returned by the subquery.
	Selecting the *IN / NOT-IN* operator in a `FilteringExpression` would create a subquery. After choosing an entity and a column to return, it checks if the value in the specified column in the outer query matches any of the values returned by the subquery.

[Otixa]

Suggested change

	Let's take a look at a practical example how the Ignite UI for Angular Query Builder Component can be used to build SQL queries.
	Let's take a look at a practical example of how the Ignite UI for Angular Query Builder Component can be used to build SQL queries.

[Otixa]

Suggested change

Let's say we want to find all suppliers who supply products that belong to the 'Beverages' category. Since the data is distributed across all entities we can take advantage of the *IN* operator and accomplish the task by creating subqueries. Each subquery is represented by a `FilteringExpressionsTree` and can be converted to a SQL query through the `transformExpressionTreeToSqlQuery(tree: IExpressionTree)` method.

Let's say we want to find all suppliers who supply products which belong to the 'Beverages' category. Since the data is distributed across all entities, we can take advantage of the *IN* operator and accomplish the task by creating subqueries. Each subquery is represented by a `FilteringExpressionsTree` and can be converted to SQL through the `transformExpressionTreeToSqlQuery(tree: IExpressionTree)` method.

[Otixa]

Suggested change

	Now we can set the `expressionsTree` property of the `IgxQueryBuilderComponent` to `suppliersTree`. Furthermore, every change to the query triggers a new request to the endpoint and the results data shown in the grid is refreshed.
	Now we can set the `expressionsTree` property of the `IgxQueryBuilderComponent` to `suppliersTree`. Furthermore, every change to the query triggers a new request to the endpoint and the resulting data shown in the grid is refreshed.