[willow] Legacy Libraries DEPR: Remove legacy libraries usage docs

[kdmccormick]

After Verawood is released (the docs repo isn't tagged for release until after Verawood goes out):

Remove all legacy libraries documentation mentions, except those which relate to the migration of legacy content to the new environment.

Future:

• [xylon] Legacy Libraries DEPR: Remove legacy libraries migration docs #1397

[sarina]

[clarification] do you mean I get to remove this whole page tree 🥹 https://docs.openedx.org/en/latest/educators/navigation/content_creation_management.html#work-with-legacy-content-libraries

If so, we should not simply delete the pages, we should delete them AND set up redirects for the pages to the https://docs.openedx.org/en/latest/educators/how-tos/course_development/migrate_legacy_libraries.html page (and alter the language on that page slightly to indicate this is required to use library content moving forward). Redirects can be set up here: https://github.com/openedx/docs.openedx.org/blob/main/source/conf.py#L60

[kdmccormick]

Yes, and sounds great 👍🏻