Host user-facing DAG Factory docs

[tatiana]

To be aligned with @cmarteepants and the docs team where we want to host the DAG Factory docs:

1. ReadTheDocs
2. Astro website
3. GitHub pages

[pankajkoti]

Since this is also related to #278 , and @pankajastro you might be aligning with @cmarteepants & the docs team on this one too, I am assigning this issue to you too. Please feel to re-assign/un-assign it later when we would actually take care of the implementation for this based on our Sprint planning.

[cmarteepants]

This is timely! Astro docs are moving away from netlify, so doesn't make sense to host on Astro until that's done. Whatever we use has to be markdown-compatible for an easy lift to the new framework, which excludes Sphinx/ReadTheDocs since that uses .rst.

Their recommendation is to use Github Pages (and markdown!)