docs: surface MathJax setup in configuration guide

[Samk13]

❤️ Thank you for your contribution!

Description

• This configuration detail was buried in the v12 "What's new" release notes and visible on search.

• Move visibility to the "Configure everything" docs where operators expect setup instructions.

• Add a link from v13 release notes to the canonical configuration section.

• to be backported to prod branch

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

Reminder

By using GitHub, you have already agreed to the GitHub’s Terms of Service including that:

1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.