feat: render READMEs in ORG format, better fallback if not MD

[mortenpi]

1. Uses Org.jl to render fallback docs from README.org files a bit better.
2. Readmes without extensions will be rendered as <pre> text.
3. Readmes that have an unknown extension will also get rendered as <pre>, but also have a warning box (content and styling TBD)

On the JuliaHub package page, Bumper (which has a README in org format) renders pretty well (although not perfectly):

The Documenter fallback might need a bit of work:

But it is better than the status quo: