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_CONV_FROM_NETWORK_FUNCTION.md

@@ -0,0 +1,114 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_CONV_FROM_NETWORK_FUNCTION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_CONV_FROM_UTF8_FUNCTION (3)
+  - CURLOPT_CONV_TO_NETWORK_FUNCTION (3)
+---
+
+# NAME
+
+CURLOPT_CONV_FROM_NETWORK_FUNCTION - convert data from network to host encoding
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode conv_callback(char *ptr, size_t length);
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CONV_FROM_NETWORK_FUNCTION,
+                          conv_callback);
+~~~
+
+# DESCRIPTION
+
+Pass a pointer to your callback function, which should match the prototype
+shown above.
+
+Applies to non-ASCII platforms. curl_version_info(3) returns the
+**CURL_VERSION_CONV** feature bit set if this option is provided.
+
+The data to be converted is in a buffer pointed to by the *ptr* parameter.
+The amount of data to convert is indicated by the *length* parameter. The
+converted data overlays the input data in the buffer pointed to by the ptr
+parameter. *CURLE_OK* must be returned upon successful conversion. A
+CURLcode return value defined by curl.h, such as *CURLE_CONV_FAILED*,
+should be returned if an error was encountered.
+
+CURLOPT_CONV_FROM_NETWORK_FUNCTION(3) converts to host encoding from the
+network encoding. It is used when commands or ASCII data are received over the
+network.
+
+If you set a callback pointer to NULL, or do not set it at all, the built-in
+libcurl iconv functions are used. If HAVE_ICONV was not defined when libcurl
+was built, and no callback has been established, the conversion returns the
+**CURLE_CONV_REQD** error code.
+
+If **HAVE_ICONV** is defined, **CURL_ICONV_CODESET_OF_HOST** must also be
+defined. For example:
+
+~~~c
+#define CURL_ICONV_CODESET_OF_HOST "IBM-1047"
+~~~
+
+The iconv code in libcurl defaults the network and UTF8 codeset names as
+follows:
+
+~~~
+#define CURL_ICONV_CODESET_OF_NETWORK "ISO8859-1"
+
+#define CURL_ICONV_CODESET_FOR_UTF8  "UTF-8"
+~~~
+
+You need to override these definitions if they are different on your system.
+
+# DEFAULT
+
+NULL
+
+# PROTOCOLS
+
+FTP, SMTP, IMAP, POP3
+
+# EXAMPLE
+
+~~~c
+static CURLcode my_conv_from_ascii_to_ebcdic(char *buffer, size_t length)
+{
+  int rc = 0;
+
+  /* in-place convert 'buffer' from ASCII to EBCDIC */
+
+  if(rc == 0) {
+    /* success */
+    return CURLE_OK;
+  }
+  else {
+    return CURLE_CONV_FAILED;
+  }
+}
+
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+
+  /* use platform-specific functions for codeset conversions */
+  curl_easy_setopt(curl, CURLOPT_CONV_FROM_NETWORK_FUNCTION,
+                   my_conv_from_ascii_to_ebcdic);
+}
+~~~
+
+# AVAILABILITY
+
+Not available and deprecated since 7.82.0.
+
+Available only if **CURL_DOES_CONVERSIONS** was defined when libcurl was
+built.
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.