Adds documentation and an integration test case for the distinct keyword in index search

[aweidner]

Pull Request

Related issue

Fixes #989

What does this PR do?

This pull requests adds documentation for the distinct keyword in the code samples file. It also adds an integration test case that exercises the distinct functionality of search.

Note that the issue mentions adding the code sample for distinct_attribute but it looks like this code sample is already in place according to the docs: https://www.meilisearch.com/docs/learn/relevancy/distinct_attribute#setting-a-distinct-attribute-during-configuration. Happy to amend this PR if that's not the case and I'm misreading this.

PR checklist

Please check if your PR fulfills the following requirements:

• [x ] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [ x] Have you read the contributing guidelines?
• [ x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[sanders41]

lgtm, thanks @aweidner!

bors merge

[meili-bors]

Build succeeded:

• integration-tests (3.10)
• integration-tests (3.11)
• integration-tests (3.12)
• integration-tests (3.8)
• integration-tests (3.9)
• mypy
• pylint