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/SSLCERTS.md

@@ -116,10 +116,10 @@ server, do one of the following:
       4. Windows Directory (e.g. C:\windows)
       5. all directories along %PATH%
 
- 5. Get a better/different/newer CA cert bundle! One option is to extract the
-    one a recent Firefox browser uses by running 'make ca-bundle' in the curl
-    build tree root, or possibly download a version that was generated this
-    way for you: [CA Extract](https://curl.se/docs/caextract.html)
+ 5. Get another CA cert bundle. One option is to extract the one a recent
+    Firefox browser uses by running 'make ca-bundle' in the curl build tree
+    root, or possibly download a version that was generated this way for you:
+    [CA Extract](https://curl.se/docs/caextract.html)
 
 Neglecting to use one of the above methods when dealing with a server using a
 certificate that is not signed by one of the certificates in the installed CA