docs: move docs to canonical.com/juju/docs/charmlibs

[dwilding]

This PR updates our doc config so that all URLs use canonical.com/juju/docs/charmlibs instead of the backend URL from Read the Docs. Our docs are still hosted on Read the Docs, but we'll be proxying them under canonical.com/juju/docs.

Preview docs built from this branch. After this PR merges, I'll switch those docs to build from main and redirect the existing docs.

Main changes

• Updated Charmlibs doc URLs in .docs/conf.py.
• Updated hardcoded URLs for Charmlibs, excluding libs that aren't maintained by Charm Tech.
• Added a script .docs/.sphinx/_static/overwrite_links.js, which is not strictly needed at the moment. This script makes sure that the version switcher from Read the Docs has correct URLs - but we don't use the version switcher. I'm including the script because it's a standard part of the new docs setup at Canonical.

Extra changes

• Updated the intersphinx URL and hardcoded URLs for Ops. To account for operator#2545
• Updated the hardcoded URLs for Jubilant. To account for jubilant#337.
• Updated the hardcoded URLs for Pebble. To account for pebble#882.

Future work (not in this PR)

• Update hardcoded URLs for Charmlibs in libs that aren't maintained by Charm Tech (one PR per team).
• Update URLs for other docs - Juju and Charmcraft.

[james-garner-canonical]

Thanks, the changes all look reasonable. Will leave it up to you to merge this when ready.