docs: introduce "curldown" for libcurl man page format

curldown is this new file format for libcurl man pages. It is markdown
inspired with differences:

- Each file has a set of leading headers with meta-data
- Supports a small subset of markdown
- Uses .md file extensions for editors/IDE/GitHub to treat them nicely
- Generates man pages very similar to the previous ones
- Generates man pages that still convert nicely to HTML on the website
- Detects and highlights mentions of curl symbols automatically (when
  their man page section is specified)

tools:

- cd2nroff: converts from curldown to nroff man page
- nroff2cd: convert an (old) nroff man page to curldown
- cdall: convert many nroff pages to curldown versions
- cd2cd: verifies and updates a curldown to latest curldown

This setup generates .3 versions of all the curldown versions at build time.

CI:

Since the documentation is now technically markdown in the eyes of many
things, the CI runs many more tests and checks on this documentation,
including proselint, link checkers and tests that make sure we capitalize the
first letter after a period...

Closes #12730

docs/CONTRIBUTE.md

@@ -116,7 +116,7 @@ features are working as they are supposed to. To maintain this situation and
 improve it, all new features and functions that are added need to be tested
 in the test suite. Every feature that is added should get at least one valid
 test case that verifies that it works as documented. If every submitter also
-posts a few test cases, it will not end up as a heavy burden on a single person!
+posts a few test cases, it will not end up as a heavy burden on a single person.
 
 If you do not have test cases or perhaps you have done something that is hard
 to write tests for, do explain exactly how you have otherwise tested and