PICARD-2103: Add custom columns documentation

[knguyen1]

Summary

This is a…

• Correction
• Addition
• Restructuring
• Minor / simple change (like a typo)
• Other

Reason for the Change

Added comprehensive documentation for the new custom columns feature in Picard. This addresses the need for user and developer documentation for the custom columns functionality that was implemented in the main Picard project.

Description of the Change

Added two new documentation files:

1. User Documentation (usage/custom_columns.rst):

   • Complete user guide for creating and managing custom columns
   • Step-by-step instructions for using the Custom Columns Manager
   • Examples for both Field Reference and Script columns
   • Best practices, troubleshooting, and common patterns
   • References to main scripting and variables documentation

2. API Documentation (extending/custom_columns_api.rst):

   • Complete developer reference for the custom columns API
   • Code examples and integration patterns
   • Factory functions, sorting adapters, and persistence utilities
   • References to implementation details

Updated navigation and index files to include the new documentation in appropriate sections.

References to implementation PRs:

• UI implementation: PICARD-2103: Add UI to manage custom columns
• API implementation: PICARD-2103: Add API for custom columns

Additional Action Required

None required. The documentation is complete and ready for use.

[rdswift]

Thanks for this. Can you please set the target branch to metabrainz:version_3_changes instead of metabrainz:master for this PR? We're trying to gather all approved Picard 3 changes into a separate branch to do a single merge when Picard 3 is released, and hopefully avoid multiple conflicts wiith each individual PR. This will also allow corrections and updates to the v2.x documentation (currently in the master branch) until we're ready to release Picard 3.

Also, how do you want to be identified in the acknowledgements page?

[knguyen1]

Thanks for this. Can you please set the target branch to metabrainz:version_3_changes instead of metabrainz:master for this PR?

Okay, I rebased and pointed it at metabrainz:version_3_changes.

Also, how do you want to be identified in the acknowledgements page?

Just my username is fine: knguyen1. Thanks.

[rdswift]

Just my username is fine: knguyen1.

Added into the master branch. It will appear the next time we publish. Thanks again.

[knguyen1]

FYI don't merge this yet. Waiting on metabrainz/picard#2714 which is still in flux.

[knguyen1]

I think you can review and merge now. The PR metabrainz/picard#2714 is pretty much done. These documentations reflect the latest changes. see: Update api/usage documentation to latest @rdswift @phw