CI: documentation deployment GitHub Action

[klauer]

• Add "extras" to blark's installation pip install blark[dev] (apischema + pytest) or pip install blark[doc] (dev deps + sphinx and such)
  • I'm considering making apischema an install requirement (#91)
• Add CI workflow for building and deploying docs
• Add lark/Python as intersphinx targets (for cross-linking API docs of lark/standard lib classes/functions)
• Use a token with codecov as it started complaining in the pytest workflow

Attempt at #90

[klauer]

Let's see if the docs deployment works. I expect I won't be so lucky on the first try.

[klauer]

I should have bought a lottery ticket instead - it worked on the first try. 🥳 Example: https://klauer.github.io/blark/master/api/blark.transform.VariableDeclarations.html