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

.github/scripts/cleanspell.pl

@@ -71,6 +71,7 @@ while(<F>) {
         $_ =~ s/curl_url_(dup)//g;
         $_ =~ s/curl_pushheader_by(name|num)//g;
         $_ =~ s/libcurl-(env|ws)//g;
+        $_ =~ s/libcurl\\-(env|ws)//g;
         $_ =~ s/(^|\W)((tftp|https|http|ftp):\/\/[a-z0-9\-._~%:\/?\#\[\]\@!\$&'()*+,;=]+)//gi;
         print O $_;
     }

.github/scripts/spellcheck.words

@@ -83,6 +83,7 @@ CCC
 CDN
 CentOS
 CFLAGS
+cflags
 CGI's
 CHACHA
 chacha
@@ -138,10 +139,11 @@ cshrc
 CTRL
 cURL
 CURLcode
+curldown
 CURLE
 CURLH
-CURLINFO
 curlimages
+CURLINFO
 curlrc
 curltest
 customizable
@@ -161,8 +163,8 @@ deepcode
 DELE
 DER
 deselectable
-Deserialized
 deserialization
+Deserialized
 destructor
 detections
 dev
@@ -350,11 +352,10 @@ interoperable
 interoperates
 IoT
 ipadOS
-IPFS
-IPNS
-ipld
-trustless
 IPCXN
+IPFS
+ipld
+IPNS
 IPv
 IPv4
 IPv4/6
@@ -804,6 +805,7 @@ TPF
 TrackMemory
 transcode
 Tru
+trustless
 Tse
 Tsujikawa
 TTL
@@ -826,6 +828,7 @@ unencoded
 unencrypted
 unescape
 Unglobbed
+Unicode
 UNICOS
 unix
 UnixSockets

.github/scripts/verify-examples.pl

@@ -91,12 +91,20 @@ sub extract {
     return ($fail ? 0 : $l);
 }
 
+my $count;
 for my $m (@files) {
-    print "Verify $m\n";
+    #print "Verify $m\n";
     my $out = extract($m);
     if($out) {
       $error |= testcompile($m);
       $error |= checksrc($m);
     }
+    $count++;
+}
+if(!$error) {
+    print "Verified $count man pages ok\n";
+}
+else {
+    print "Detected problems\n";
 }
 exit $error;