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/CURLOPT_COOKIEFILE.md

@@ -0,0 +1,103 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_COOKIEFILE
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_COOKIE (3)
+  - CURLOPT_COOKIEJAR (3)
+  - CURLOPT_COOKIESESSION (3)
+---
+
+# NAME
+
+CURLOPT_COOKIEFILE - file name to read cookies from
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_COOKIEFILE, char *filename);
+~~~
+
+# DESCRIPTION
+
+Pass a pointer to a null-terminated string as parameter. It should point to
+the file name of your file holding cookie data to read. The cookie data can be
+in either the old Netscape / Mozilla cookie data format or just regular HTTP
+headers (Set-Cookie style) dumped to a file.
+
+It also enables the cookie engine, making libcurl parse and send cookies on
+subsequent requests with this handle.
+
+By passing the empty string ("") to this option, you enable the cookie engine
+without reading any initial cookies. If you tell libcurl the file name is "-"
+(just a single minus sign), libcurl instead reads from stdin.
+
+This option only **reads** cookies. To make libcurl write cookies to file,
+see CURLOPT_COOKIEJAR(3).
+
+If you read cookies from a plain HTTP headers file and it does not specify a
+domain in the Set-Cookie line, then the cookie is not sent since the cookie
+domain cannot match the target URL's. To address this, set a domain in
+Set-Cookie line (doing that includes subdomains) or preferably: use the
+Netscape format.
+
+If you use this option multiple times, you add more files to read cookies
+from.
+
+The application does not have to keep the string around after setting this
+option.
+
+Setting this option to NULL (since 7.77.0) explicitly disables the cookie
+engine and clears the list of files to read cookies from.
+
+# SECURITY
+
+This document previously mentioned how specifying a non-existing file can also
+enable the cookie engine. While true, we strongly advise against using that
+method as it is too hard to be sure that files that stay that way in the long
+run.
+
+# DEFAULT
+
+NULL
+
+# PROTOCOLS
+
+HTTP
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    CURLcode res;
+    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/foo.bin");
+
+    /* get cookies from an existing file */
+    curl_easy_setopt(curl, CURLOPT_COOKIEFILE, "/tmp/cookies.txt");
+
+    res = curl_easy_perform(curl);
+
+    curl_easy_cleanup(curl);
+  }
+}
+~~~
+
+# Cookie file format
+
+The cookie file format and general cookie concepts in curl are described
+online here: https://curl.se/docs/http-cookies.html
+
+# AVAILABILITY
+
+As long as HTTP is supported
+
+# RETURN VALUE
+
+Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not.