feat(grids): add cell merging topic

[IMinchev64]

Needs this PR to be reviewed first.

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

[ChronosSF]

@IMinchev64 I merged the samples PR but I believe there will need to be 2 samples each for tree grid and hierarchical grid, not 1 like now.

[dkamburov]

@IMinchev64 the master-detail feature and the limititaion about MRL shouldn't be part of tree-grid or hrid topics as they are not supported there: