Sphinx Multiversioning

[ZergLev]

Description

Added Sphinx Multiversioning utilising JamesIves/github-deploy-action's feature of saving commit history on gh-pages branch.

You can see a working example here: https://zerglev.github.io/chatsky/master/index.html. Use the version switcher button in the top-right to switch between versions.
Major changes:

• Can build and deploy documentation for any pre-configured branch or tag. We can build docs for versions up to v0.8.0; versions before that are too different. (due to renaming 'DFF' -> 'Chatsky')
• Has a version switcher button. (in top right corner) It only shows master (as 'latest'), dev and all tags like v0.9.0 from a specified 'start_version'.
• Documentation for different versions is saved in separate folders, and you can conveniently switch the folder to view the same file for a different version (if it exists)

Minor details:

• In case there is no version selected for docs, say, someone used an old link, like https://deeppavlov.github.io/chatsky/index.html, the website will redirect them to master by default.
• In case someone makes a mistake and types /latest/ within their docs link, it will redirect them to /master/.
• All relevant links are version-dependent.

Checklist

• Made a working example with master and dev built separately
• Version switcher works correctly
• Using tags to create docs for different versions
• Pull Requests build docs correctly
• Deleting old builds before deploying new builds
• Made all relevant links version-dependent
• Redirects work now
• I have performed a self-review of the changes
• Tested local doc builds
• Receive a review and do the required changes

To Consider

• We could redirect people to chatsky in case someone types dialog_flow_framework, it's very easy to do.
• Testing Github Pages deployment from main Chatsky repo.
• Testing documentation building from Chatsky repo.
• SEO optimization (old links will show the wrong website, even though they will get redirected now) (just need to add a few lines into index.html if we want to do that)
• Finding a better place/design for the version switcher button. (It's maybe okay now, actually)
• Update CONTRIBUTING.md (if devel workflow is changed)
• Update .ignore files, scripts (such as lint), distribution manifest (if files are added/deleted)

Notes:

• "git stash" and "git checkout" are used during the Github Workflow. I'm not sure if that poses a problem, but everything is returned to it's place in the end, and I don't really know how to do things otherwise. It might just be required. If other workflows just wait for this one, then there can't be a problem, I think. Or if they don't run at the same time.
• We're always using old conf.py files, meaning the docs will be exactly the same way, they were before - all brand names, pictures, website designs might be outdated.

[RLKRo]

I also approved the run of github actions which returned a couple of errors.
AFAIK pushing new commits here would now retrigger github CI so you can see if the errors are gone.

[RLKRo]

• Redirect chatsky/path to chatsky/master/path (maybe also rename to latest).
• Do not use patched polyversion -- pypi doesn't accept that. Can patch sys.argv in scripts/docs.py.
• Version switcher displays "dev" when on dev but "Choose version" when on master.

[RLKRo]

Fix tutorial links:

Colab link does not contain version;
github link always links to master.