source-files: Document markdown processing behaviour

[schickling]

• MDX: we're using mdx-bundler
• Markdown: we're using unified with these plugins by default

[seancdavis]

Noting that much has changed in this behavior in 0.2.5, and we should have a page for both Markdown and MDX that go into detail about the various options and patterns available.

We should also ensure we're linking to these pages from the technical reference.

This video has more context: https://cln.sh/I10Kzq

[seancdavis]

MDX page can be found here: https://www.contentlayer.dev/docs/sources/files/mdx

Markdown doc still needs to be written. And it'd be nice to have additional pages for JSON and YAML, just as a place to add syntax-specific notes.

Address these issues as part of this work:

• Passing raw HTML tags through the markdown processor #43
• Add a list of supported document types for source-files #46