Create a "Get started" page

That allows users to quickly start playing with Validation.

Author: henriquemoody

docs/get-started.md

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+---
+weight: -100
+---
+
+# Get started
+
+Welcome to Respect\Validation!
+
+This guide will help you get up and running with the library quickly.
+
+## Installation
+
+To install Respect\Validation, use [Composer](http://getcomposer.org):
+
+```shell
+composer require respect/validation:^3.0
+```
+
+Ensure you have PHP 8.5 or above installed.
+
+## Basic usage
+
+The `ValidatorBuilder` (aliased as `v` for convenience) provides a fluent interface for building validators and running them.
+
+### Validating using exceptions
+
+The `assert()` method throws an exception when validation fails. Handle these exceptions with `try/catch` for robust error handling:
+
+```php
+try {
+    v::intType()->assert($input);
+} catch (Throwable $exception) {
+    echo 'Validation failed: ' . $exception->getMessage();
+}
+```
+
+### Validating without exceptions
+
+The `validate()` method returns a `ResultQuery` object that allows you to inspect and display validation results:
+
+```php
+$result = v::intType()->validate($input);
+if (!$result->isValid()) {
+    echo 'Validation failed: ' . $result->getMessage();
+}
+```
+
+### Validating using booleans
+
+Use the `isValid()` method to check if your input meets specific validation criteria:
+
+```php
+if (!v::intType()->isValid($input)) {
+    echo 'The input you gave me is not an integer';
+}
+```
+
+## Key Features
+
+### Complex validation
+
+Combine multiple validators for complex validation rules:
+
+```php
+v::numericVal()->positive()->between(1, 255)->assert($input);
+```
+
+### Custom error messages
+
+Define your own error messages when validation fails:
+
+```php
+v::between(1, 256)->assert($input, '{{subject}} is not what I was expecting');
+```
+
+### Custom exceptions
+
+Throw your own exceptions when the validation fails:
+
+```php
+v::between(1, 256)->assert($input, new DomainException('Not within the expected range'));
+```
+
+### Reusing validators
+
+Create validators once and reuse them across multiple inputs:
+
+```php
+$validator = v::alnum()->lowercase();
+
+$validator->assert('respect');
+$validator->assert('validation');
+```
+
+## Next steps
+
+- Explore the [Feature Guide](feature-guide.md) for more advanced usage.
+- Check out the [List of Validators by Category](list-of-validators-by-category.md) for a comprehensive list of available validators.