Update Liquid Tag/Markdown Documentation

[michael-tharrington]

Explain the changes

This one isn't so straightforward. We need to:

1. Update our Liquid Tag docs to be in line with the new Unified Liquid Embed project as discussed in this post.
2. Remove existing duplicated documentation (e.g., don't repeat knowledge)
3. Determine the best places to put this documentation

Suggested changes

Not a straightforward update to our admin docs. This is instead about updating user facing docs.

Additional context

Whatever user-facing docs we create here should be updated across all Forems.

[afrinashar]

I would like to work on this.