Add EAV antipattern to database docs

What does this MR do?

This MR adds a section to our database documentation about the EAV antipattern.

This was brought up on a discussion with @abrandl !25199 (comment 307886442).

I did not know about this pattern, so maybe others will learn about it if we start to document it.

This is still a WIP MR that we will need to iterate on it. @abrandl as I'm not an expert with database do you think this would be helpful to other folks? If yes, could you help me to write down what readers need to be aware of when looking at this page? 🙏

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