Documentation for marking files as viewed

What does this MR do?

For #17531 (closed)

This MR is just the documentation for the feature described in the above issue and implemented in !51513 (merged).
It also flips the feature flag to default to true instead of default to false.

To test the feature itself, check out !51513 (merged) and enable the feature with Feature.enable(:local_file_reviews).

Please note the Use the feature and Notable caveats sections of that merge request.

Merge Request	Description
!49173 (merged) !49174 (merged) !49190 (merged) !49208 (merged)	Various cleanup merge requests in preparation for this feature.
!49506 (merged)	Identify diff files using the content_sha
!50022 (merged)	Switch identification to use blob.id
!50033 (merged)	Add tooling to add/remove reviews from LocalStorage to accommodate the file review feature
!50132 (merged)	Add the current merge request IID to the file ID
!51513 (merged)	Add UI to review files
We're here 👉🏻	Add documentation for file reviews

Screenshots (strongly suggested)

N/A: docs

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team