[WIP][POC] Proof of concept for documentation refactor.

[hogepodge]

Proposal for documentation refactor. Changes include:

Major changes to top-level structure.
Separation of user and developer docs.
Reclassifying many tutorials as how-tos.
Moving sphinx galleries to top level "gallery" directory.
Consistent naming between galleries and top-level structure

do not merge

[Lunderberg]

@areusch Do the CI nodes get fully cleaned before starting a new task, or do they just check out the PR's version?

If the latter, I think this PR may be causing unrelated PRs to fail CI. The CI gave errors about duplicate files existing in both the docs/user_tutorials folder existing and having duplicate labels as compared to other .rst files.

[mbs-octoml]

If the latter, I think this PR may be causing unrelated PRs to fail CI.

Dito for https://ci.tlcpack.ai/blue/organizations/jenkins/tvm/detail/PR-8597/40/pipeline/257/

[hogepodge]

One reason for the failures may be that I'm actually duplicating files at this point as I'm trying to sort out a new structure.

[Lunderberg]

Just put in #8735, which (crossing fingers) should resolve the CI issues.

[mbs-octoml]

For posterity: The sphinx build was placing build files directly into the source tree. We rely on git checkout to keep the source tree 1:1 with the PR under test and thus hermetic, but this build trick broke that assumption.