Document the possible issue of searching binary file

What does this MR do?

This is related to #301083 (closed) and gitlab-elasticsearch-indexer#72 (closed). We have added a change where all the files are searchable by their names, gitlab-elasticsearch-indexer!99 (merged). However, if users do not re-indexing everything from scratch again, only newly added or updated binary files can be found. The MR is to document this known issue.

I also restructured the documentation a little bit.

Screenshots (strongly suggested)

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