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_WILDCARDMATCH.md

@@ -0,0 +1,115 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_WILDCARDMATCH
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_CHUNK_BGN_FUNCTION (3)
+  - CURLOPT_CHUNK_END_FUNCTION (3)
+  - CURLOPT_FNMATCH_FUNCTION (3)
+  - CURLOPT_URL (3)
+---
+
+# NAME
+
+CURLOPT_WILDCARDMATCH - directory wildcard transfers
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WILDCARDMATCH, long onoff);
+~~~
+
+# DESCRIPTION
+
+Set *onoff* to 1 if you want to transfer multiple files according to a
+file name pattern. The pattern can be specified as part of the
+CURLOPT_URL(3) option, using an **fnmatch**-like pattern (Shell
+Pattern Matching) in the last part of URL (file name).
+
+By default, libcurl uses its internal wildcard matching implementation. You
+can provide your own matching function by the
+CURLOPT_FNMATCH_FUNCTION(3) option.
+
+A brief introduction of its syntax follows:
+
+## * - ASTERISK
+
+~~~c
+  ftp://example.com/some/path/*.txt
+~~~
+for all txt's from the root directory. Only two asterisks are allowed within
+the same pattern string.
+
+## ? - QUESTION MARK
+
+Question mark matches any (exactly one) character.
+~~~c
+  ftp://example.com/some/path/photo?.jpg
+~~~
+
+## [ - BRACKET EXPRESSION
+
+The left bracket opens a bracket expression. The question mark and asterisk have
+no special meaning in a bracket expression. Each bracket expression ends by the
+right bracket and matches exactly one character. Some examples follow:
+
+**[a-zA-Z0-9]** or **[f-gF-G]** - character interval
+
+**[abc]** - character enumeration
+
+**[^abc]** or **[!abc]** - negation
+
+**[[:name:]]** class expression. Supported classes are
+**alnum**,**lower**, **space**, **alpha**, **digit**, **print**,
+**upper**, **blank**, **graph**, **xdigit**.
+
+**[][-!^]** - special case - matches only '-', ']', '[', '!' or '^'. These
+characters have no special purpose.
+
+**[[]]** - escape syntax. Matches '[', ']' or 'e'.
+
+Using the rules above, a file name pattern can be constructed:
+~~~c
+  ftp://example.com/some/path/[a-z[:upper:]\\].jpg
+~~~
+
+# PROTOCOLS
+
+This feature is only supported for FTP download.
+
+# EXAMPLE
+
+~~~c
+
+extern long begin_cb(struct curl_fileinfo *, void *, int);
+extern long end_cb(void *ptr);
+
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    /* turn on wildcard matching */
+    curl_easy_setopt(curl, CURLOPT_WILDCARDMATCH, 1L);
+
+    /* callback is called before download of concrete file started */
+    curl_easy_setopt(curl, CURLOPT_CHUNK_BGN_FUNCTION, begin_cb);
+
+    /* callback is called after data from the file have been transferred */
+    curl_easy_setopt(curl, CURLOPT_CHUNK_END_FUNCTION, end_cb);
+
+    /* See more on https://curl.se/libcurl/c/ftp-wildcard.html */
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.21.0
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.