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_INTERFACE.md

@@ -0,0 +1,83 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_INTERFACE
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_SOCKOPTFUNCTION (3)
+  - CURLOPT_TCP_NODELAY (3)
+---
+
+# NAME
+
+CURLOPT_INTERFACE - source interface for outgoing traffic
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_INTERFACE, char *interface);
+~~~
+
+# DESCRIPTION
+
+Pass a char pointer as parameter. This sets the *interface* name to use as
+outgoing network interface. The name can be an interface name, an IP address,
+or a host name.
+
+If the parameter starts with "if!" then it is treated only as an interface
+name. If the parameter starts with &"host!" it is treated as either an IP
+address or a hostname.
+
+If "if!" is specified but the parameter does not match an existing interface,
+*CURLE_INTERFACE_FAILED* is returned from the libcurl function used to
+perform the transfer.
+
+libcurl does not support using network interface names for this option on
+Windows.
+
+We strongly advise against specifying the interface with a hostname, as it
+causes libcurl to do a blocking name resolve call to retrieve the IP
+address. That name resolve operation does **not** use DNS-over-HTTPS even if
+CURLOPT_DOH_URL(3) is set.
+
+The application does not have to keep the string around after setting this
+option.
+
+# DEFAULT
+
+NULL, use whatever the TCP stack finds suitable
+
+# PROTOCOLS
+
+All
+
+# 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");
+
+    curl_easy_setopt(curl, CURLOPT_INTERFACE, "eth0");
+
+    res = curl_easy_perform(curl);
+
+    curl_easy_cleanup(curl);
+  }
+}
+~~~
+
+# AVAILABILITY
+
+The "if!" and "host!" syntax was added in 7.24.0.
+
+# RETURN VALUE
+
+Returns CURLE_OK on success or
+CURLE_OUT_OF_MEMORY if there was insufficient heap space.