Static page extend html sanitization

[Samk13]

:heart: Thank you for your contribution!

Description

Add docs on how to extend Static page sanitization on the instance level for V12. This PR is documentation for this

Checklist

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

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [x] I've added relevant test cases.
• [x] I've added relevant documentation.
• [x] I've marked translation strings (for relevant code).
• [x] I've followed the CSS/JS and React guidelines (for relevant code).
• [x] I've followed the web accessibility guidelines (for relevant code).
• [x] I've followed the user interface guidelines (for relevant code).
• [x] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [x] I've NOT included third-party code (copy/pasted source code or new dependencies).

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[fenekku]

I will start to look into the material docs deprecation warnings shortly to help resolve build issues: https://github.com/inveniosoftware/docs-invenio-rdm/issues/591 . Thanks for your patience!

[fenekku]

mkdocs upgrade PR: https://github.com/inveniosoftware/docs-invenio-rdm/pull/592 . Once it's merged, contributions should pass again.