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_HSTSWRITEFUNCTION.md

@@ -0,0 +1,110 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_HSTSWRITEFUNCTION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_HSTS (3)
+  - CURLOPT_HSTSWRITEDATA (3)
+  - CURLOPT_HSTSWRITEFUNCTION (3)
+  - CURLOPT_HSTS_CTRL (3)
+---
+
+# NAME
+
+CURLOPT_HSTSWRITEFUNCTION - write callback for HSTS hosts
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+struct curl_hstsentry {
+  char *name;
+  size_t namelen;
+  unsigned int includeSubDomains:1;
+  char expire[18]; /* YYYYMMDD HH:MM:SS [null-terminated] */
+};
+
+struct curl_index {
+  size_t index; /* the provided entry's "index" or count */
+  size_t total; /* total number of entries to save */
+};
+
+CURLSTScode hstswrite(CURL *easy, struct curl_hstsentry *sts,
+                      struct curl_index *count, void *clientp);
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSWRITEFUNCTION, hstswrite);
+~~~
+
+# DESCRIPTION
+
+Pass a pointer to your callback function, as the prototype shows above.
+
+This callback function gets called by libcurl repeatedly to allow the
+application to store the in-memory HSTS cache when libcurl is about to discard
+it.
+
+Set the *clientp* argument with the CURLOPT_HSTSWRITEDATA(3) option
+or it is NULL.
+When the callback is invoked, the *sts* pointer points to a populated
+struct: Read the host name to 'name' (it is *namelen* bytes long and null
+terminated. The *includeSubDomains* field is non-zero if the entry matches
+subdomains. The *expire* string is a date stamp null-terminated string
+using the syntax YYYYMMDD HH:MM:SS.
+
+The callback should return *CURLSTS_OK* if it succeeded and is prepared to
+be called again (for another host) or *CURLSTS_DONE* if there is nothing
+more to do. It can also return *CURLSTS_FAIL* to signal error.
+
+This option does not enable HSTS, you need to use CURLOPT_HSTS_CTRL(3) to
+do that.
+
+# DEFAULT
+
+NULL - no callback.
+
+# PROTOCOLS
+
+This feature is only used for HTTP(S) transfer.
+
+# EXAMPLE
+
+~~~c
+struct priv {
+  void *custom;
+};
+
+static CURLSTScode hswr_cb(CURL *easy, struct curl_hstsentry *sts,
+                           struct curl_index *count, void *clientp)
+{
+  /* save the passed in HSTS data somewhere */
+  return CURLSTS_OK;
+}
+
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    struct priv my_stuff;
+    CURLcode res;
+
+    /* set HSTS read callback */
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEFUNCTION, hswr_cb);
+
+    /* pass in suitable argument to the callback */
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEDATA, &my_stuff);
+
+    res = curl_easy_perform(curl);
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.74.0
+
+# RETURN VALUE
+
+This returns CURLE_OK.