This PR adds all the juju.is/docs content featuring Charmcraft.

The content was originally formatted for Discourse; this PR reformats it to serve the RTD project the docs are destined for.

The content was also originally entangled with Ops content; this PR significantly refactors -- as well as clarifies and tries to complete -- everything to serve the stand-alone Charmcraft docs set it is destined for.

Issues that should be fixed in subsequent PRs:

• Everywhere: (1) (a) Links to juju.is will work, as even after moving docs out of there we'll have redirects; however, they should be updated. (b) Links to ops docs are empty -- they reference content I'm planning to move there post-refactoring; once we have them in place, which should be in the next few days and pre-official-Charmcraft-RTD-launch, we should add them in Charmcraft docs.
• Tutorial: The tutorial should also be offered in the "regular" flavor, that is, for apps that are not 12-factor. That flavor should also feature the publish step. (See, e.g., our old Kubernetes tutorial, which includes https://juju.is/docs/sdk/publish-your-charm-on-charmhub . PS If the Charmcraft tutorial starts featuring this, as it should, the old Kubernetes tutorial, which will be moved to the stand-alone Ops docs, can focus on just development, which is what Ops is really all about.)
• How-to guides: While this PR surfaces a lot more of Charmcraft and arguably a lot more clearly, there are still corners with cobwebs or gaps. These include: (1) Manage charms (12-factor app) -- I think this doc should be incorporated into the main Manage charms doc, which could have tabs for "regular', "django", etc. (This would align with plans for the juju.is/docs entrypoint into the "federated docs"). (2) The Manage parts doc, which is full of "TBA" -- i.e., has gaps. (3) The docs in "Misc", which are not well integrated with everything else here. (4) Maybe the Manage bundles doc, which doesn't by far tell the full story; given that we're phasing bundles out, it's also perhaps fine not to worry about this too much. (5) Commands analyse and test are not featured in any how-to guide.
• Reference: (1) The autogenerated charmcraft CLI command reference docs should have anchors on the template command-charmcraft-x and titles on the template "Command charmcraft x. This will not better align with existing practices in the Juju world but also results in more clarity when linking to the page using {ref}...`` (2) The various file reference docs should be as much as possible autogenerated from source (likely the source will have to be updated first) -- at the very least, file charmcraft.yaml. They may also need further clean-up. (3) The part docs are a bit of a mess. We should move any relevant content to the part docs, then link up to that and only keep in Charmcraft the material specific to Charmcraft docs, and then revisit that material as well to streamline it as much as possible (e.g., currently the dump plugin is documented in 3 places). (4) Charmcraft analyzers and linters -- this gives details on pack and analyse but it's not clear in what way it's useful to the reader, so we need to rethink.
• Explanation: We have nothing there. I didn't delete that section as I think there is potentially a topic we could have there -- e.g., Charmcraft in the bigger context of Starcraft (a clarification of the general design philosophy there and trends, maybe).

PS Some docs may have a Contributors list on the bottom, listing the Discourse handles of the contributors to date. We'll want to rethink how we do that.