Complete reference for the Directive file properties

[flaki]

Something I've been frequently bumping into is writing non-trivial Directive options manually and getting bogged down by our docs. The Dockerfile reference I think provides a really clean, well, reference for how I imagine this should work for reducing the barrier of entry and Docusaurus already works very similarly to that page (giving us a TOC on the right hand side for easy navigation, which we could use similarly to how it is on the Dockerfile ref pages).

[LauraLangdon]

@flaki Do we still need this now that we're not drawing users' attention to the Directive?