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/libcurl/opts/CURLINFO_PROTOCOL.md

@@ -0,0 +1,73 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLINFO_PROTOCOL
+Section: 3
+Source: libcurl
+See-also:
+  - CURLINFO_RESPONSE_CODE (3)
+  - curl_easy_getinfo (3)
+  - curl_easy_setopt (3)
+---
+
+# NAME
+
+CURLINFO_PROTOCOL - get the protocol used in the connection
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_PROTOCOL, long *p);
+~~~
+
+# DESCRIPTION
+
+This option is deprecated. We strongly recommend using
+CURLINFO_SCHEME(3) instead, because this option cannot return all
+possible protocols!
+
+Pass a pointer to a long to receive the version used in the last http
+connection. The returned value is set to one of the CURLPROTO_* values:
+
+~~~c
+CURLPROTO_DICT, CURLPROTO_FILE, CURLPROTO_FTP, CURLPROTO_FTPS,
+CURLPROTO_GOPHER, CURLPROTO_HTTP, CURLPROTO_HTTPS, CURLPROTO_IMAP,
+CURLPROTO_IMAPS, CURLPROTO_LDAP, CURLPROTO_LDAPS, CURLPROTO_POP3,
+CURLPROTO_POP3S, CURLPROTO_RTMP, CURLPROTO_RTMPE, CURLPROTO_RTMPS,
+CURLPROTO_RTMPT, CURLPROTO_RTMPTE, CURLPROTO_RTMPTS, CURLPROTO_RTSP,
+CURLPROTO_SCP, CURLPROTO_SFTP, CURLPROTO_SMB, CURLPROTO_SMBS, CURLPROTO_SMTP,
+CURLPROTO_SMTPS, CURLPROTO_TELNET, CURLPROTO_TFTP, CURLPROTO_MQTT
+~~~
+
+# PROTOCOLS
+
+All
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    CURLcode res;
+    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+    res = curl_easy_perform(curl);
+    if(res == CURLE_OK) {
+      long protocol;
+      curl_easy_getinfo(curl, CURLINFO_PROTOCOL, &protocol);
+    }
+    curl_easy_cleanup(curl);
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.52.0. Deprecated since 7.85.0.
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.