Update database guides on text data type usage

What does this MR do?

This MR adds documentation on how to use the text data type in migrations:

1. Using text vs string
2. How to add text columns and set their limits
3. How to split migrations that include add_text_limit
4. How to update existing columns and run a data migration to clean up existing invalid records

Related MRs:

• Add migration helpers for managing check constraints and limits on text columns
• Enforce text columns with a limit over strings on migrations
• Prevent add_text_limit to be used with validate: true on tables with invalid data

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

Closes #215065 (closed), #214256 (closed)