docs: replace previous docs with sphinx docs

[AloizioMacedo]

This PR replaces the previous documentation with the new one, which uses sphinx and leverages read the docs as hosting platform. As part of this, it does the last step of replacing the offline docs (it is convenient to do both at once when changing snapcraft.yaml, since we will not need to temporarily build two docs on the snap).

Before opening for review, this will wait for:

• Formal approval on the spec, since it is a destructive process. Done
• Other PRs that implement things related to the transition spec. As of this moment, docs: automate cli docs generation #144. Done

This will also finalize the transition process.

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Tips on reviewing: Commits remove docs folder, remove offline-docs, rename rtd-docs to docs should be reviewed as high level changes, as they are only overarching removals and renamings.

[wyattrees]

+1, although it looks like some tests need to either be removed or refactored.

Also, if I'm not mistaken, the description of the PR gets included in the commit message when merge happens, so make sure that it complies with our conventional commits spec