Convert website to Sphinx

[speth]

Changes

• Move the "outer" non-versioned portion of the website from Nikola to Sphinx, providing a consistent visual appearance
  • Closes #209
• Convert pages from ReST to MyST Markdown format
  • Closes #211
• Remove content that has been migrated into the versioned documentation
  • Closes https://github.com/Cantera/enhancements/issues/178
• Adjust paths to different pages to shorten URLs and to make it possible to create links to "stable" and "dev" docs without picking a specific version
  • Resolves #229.

TODO

• [ ] Update CI configuration to run Sphinx

A version of the website built from this is available for exploration at https://testing.cantera.org. Since the new section layout is only currently available in the "dev" docs, this site has the "stable" docs replaced with the "dev" docs.

I think the plan for this PR should be to just push it as a separate 3.1 branch once people have had a chance to review the changes. After that, any updates to the main branch will need to be replicated here, and this branch will be merged with the main branch at the time of the Cantera 3.1 release, with all merge conflicts resolved in favor of this branch.

(To clarify: I think this is ready for review; It is marked as "draft" to prevent accidentally merging, though I guess that's not really an issue given the merge conflicts)

[bryanwweber]

@speth Do you mind if I force-push this branch to remove my work email from the commits? Just want to check so you know when to reset --hard 😄

[bryanwweber]

@speth Do you mind if I force-push this branch to remove my work email from the commits? Just want to check so you know when to reset --hard 😄

@speth all set, thank you!

[speth]

From my end, I had a brief look at testing.cantera.org, but at the moment it still appears to be the Nikola theme? Or is it an exact clone in terms of appearance with the backend replaced?

Sorry, looks like I had messed something up and what I had deployed got clobbered. I think it should be working now (and shouldn't get overwritten unless something gets pushed to the testing branch).