Don't support installation via Git

[agjohnson]

Currently, we don't officially support installation via Git, but projects that are attempting to install via Git were required to build assets, via our build_py override. See #1037 and the #1039 temporary fix for some background.

We hit a number of issues supporting installation from Git:

• We have no control over packaging
• Users get an inconsistent experience, as core team treats releases as canonical versions, not master branch
• PR merge conflicts on every PR and PR change
• Installation via Git is mostly historical, we should solve user issues with unreleased fixes using development releases.

#1039 temporarily reverted customization of the package build process. There was a source package issue also in #1014 that is unrelated to removing package build override in #1039.

Work

• Create a warning about Git installation deprecation. Detect when building from source or when using a non-release version of the theme. Perhaps there is something we can do with package version here?
• Document deprecation
  • Notes on deprecation change
  • How we recommend using the theme for given use cases:
    • Users installing as a submodule to override SASS or for some unknown reason
      • Maintain your own fork against a Git tag, not master
    • Users installing via Git directly to get latest branch
      • Use RC packages instead
• Release plan for deprecation

Design decisions

• How to technically detect a direct installation at Sphinx build time?
• What additional use cases are we concerned about?
• How do we release backwards incompatible changes?
  • Initial change to announce deprecation
  • Final deprecation. Do we lump this in with Bootstrap changes?

[Blendify]

or when using a non-release version of the theme.

This is possible with

https://github.com/readthedocs/sphinx_rtd_theme/pull/813/files

[Blendify]

I think for 1.0 we want to only deprecate, we can fully remove built assets in 2.0.

Is #813 good enough as a warning to users?

[agjohnson]

I think for 1.0 we want to only deprecate, we can fully remove built assets in 2.0.

Yeah, agreed. I added 1.0 here to get the deprecation warning added, but we can address that starting with #813