Add DocsDocumenter action

[penelopeysm]

This PR adds a second action in which builds Documenter.jl docs, then insert the navbar before deploying.

Usage in another repo would just then be

on:
  push:
    branches: [main]

jobs:
  build-docs:
    permissions:
      contents: write
    runs-on: ubuntu-latest

    steps:
      - name: Build Documenter.jl docs
        uses: TuringLang/DocsNav/DocsDocumenter

I am pretty sure that this should work across pretty much every TuringLang repo!

@shravanngoswamii This isn't a complete PR, just a proof of principle – but if you agree with the principle of doing this, then I can:

• rename the repo just to 'actions' (then it can become a catch-all for all the actions we want to have)
• update the README accordingly (this pr is shockingly underdocumented 😄)

[shravanngoswamii]

but if you agree with the principle of doing this

To be honest, I never thought of doing it this way😅, but it's a great idea! This will help us maintain consistency in building docs for all repos! When I was writing initial DocsNav scripts, I noticed that many repo's uses different and old version of Documenter, this approach will fix this too! Let's do it!

[penelopeysm]

Okay. I'll merge this, tag a new version and then test it out on AbstractPPL :)

[penelopeysm]

TuringLang/AbstractPPL.jl#110 😄