shader-slang.github.io: Set up actions to sync submodules and Slang release tags for readthedocs

[aidanfnv]

For the readthedocs documentation, the plan is to have different versions of the docs, at least 2:

• A stable version, tied to the latest release of Slang
• A development version, tied to the master branch of Slang

Github actions should be set up in this repository to create a branch here for the latest Slang release, and to update the submodule in that branch to sync with whatever is the latest release of Slang when the action runs.
readthedocs can then be configured to build that stable branch whenever the branch is pushed to, so when a new release tag is created, the action should update the stable branch to track that release and update the submodule to checkout the tag and push back to this repo, triggering a readthedocs rebuild.
The other submodules will also need to be periodically updated as well.

[aidanfnv]

There is an issue with the webhooks, reopening