Allow markdown in collection descriptions

**Is your feature request related to a problem? Please describe.**
I want to provide better instructions and links in the descriptions of collections.

**Describe the solution you'd like**
Enable markdown parsing in collection descriptions, as with field hints.

**Describe alternatives you've considered**
plain text of urls to docs

**Additional context**
https://github.com/netlify/netlify-cms/pull/5584 but for collection descriptions