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_NETRC_FILE.md

@@ -0,0 +1,66 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_NETRC_FILE
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_NETRC (3)
+  - CURLOPT_PASSWORD (3)
+  - CURLOPT_USERNAME (3)
+---
+
+# NAME
+
+CURLOPT_NETRC_FILE - file name to read .netrc info from
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_NETRC_FILE, char *file);
+~~~
+
+# DESCRIPTION
+
+Pass a char pointer as parameter, pointing to a null-terminated string
+containing the full path name to the *file* you want libcurl to use as .netrc
+file. If this option is omitted, and CURLOPT_NETRC(3) is set, libcurl checks
+for a .netrc file in the current user's home directory.
+
+The application does not have to keep the string around after setting this
+option.
+
+# DEFAULT
+
+NULL
+
+# PROTOCOLS
+
+All
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    CURLcode ret;
+    curl_easy_setopt(curl, CURLOPT_URL, "ftp://example.com/");
+    curl_easy_setopt(curl, CURLOPT_NETRC, CURL_NETRC_OPTIONAL);
+    curl_easy_setopt(curl, CURLOPT_NETRC_FILE, "/tmp/magic-netrc");
+    ret = curl_easy_perform(curl);
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.10.9
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
+CURLE_OUT_OF_MEMORY if there was insufficient heap space.