Gracefully deprecate legacy docs

[pedro-psb]

Progress

Some repositories didn't had doc pages before, so this task doesn't apply.

plugins

• Add legacy docs deprecation warning on every page plugin_template#860 (will auto update enabled plugins)
• Fix docs deprecation notice pulp_rpm#3518 (ci_update_docs: false)
• Add deprecation note to legacy docs pulp_python#664 (ci_update_docs: false)

others

• Add docs migration warning and redirects from old pages pulp-cli#949
• Add deprecation banner for old docs pulp-oci-images#631
• Add deprecation banner to old docs pulp-operator#1265

Approach

A) Banner with link + schedule deletion/freeze (choosed)

• Link to new docs (on the appropriate repo page)
• Inform what will happen to the legacy website (either be deleted after some day or remain freezed).

Pro: simple and low cost
Cons: people with legacy links need to find the correspondent fresh content on the new docs

B) Per-Page Redirects (discarded)

• Remove old legacy content
• Create per-page redirects to matching content (no banner necessary in this case)

Pros: require more work
Cons: people with legacy links will go directly to fresh content

[pedro-psb]

After some brief discussion with @decko, @dkliban and @bmbouter , we've decided the following as a general strategy:

• Schedule a redirection of for July 2024:
  • http://docs.pulpproject.org/pulp* to pulpproject.org (which will contain the current staging).
  • Dont redirect pulp2 docs (https://docs.pulpproject.org/en/2.21/), which should remain on air.
• For now, add banners to websites with schedule information and a link to the new website.

[pedro-psb]

As an small addition to this idea, instead of redirecting to the pulpproject.org home, we should redirect to a "transition section" with some minimum background and useful tips for using the new docs (such as "Help > Documentation Usage" page, which is currently a only placeholder)