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_HTTP_VERSION.md

@@ -0,0 +1,119 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_HTTP_VERSION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_ALTSVC (3)
+  - CURLOPT_HTTP09_ALLOWED (3)
+  - CURLOPT_HTTP200ALIASES (3)
+  - CURLOPT_SSLVERSION (3)
+---
+
+# NAME
+
+CURLOPT_HTTP_VERSION - HTTP protocol version to use
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTP_VERSION, long version);
+~~~
+
+# DESCRIPTION
+
+Pass *version* a long, set to one of the values described below. They ask
+libcurl to use the specific HTTP versions.
+
+Note that the HTTP version is just a request. libcurl still prioritizes to
+reuse existing connections so it might then reuse a connection using a HTTP
+version you have not asked for.
+
+## CURL_HTTP_VERSION_NONE
+
+We do not care about what version the library uses. libcurl uses whatever it
+thinks fit.
+
+## CURL_HTTP_VERSION_1_0
+
+Enforce HTTP 1.0 requests.
+
+## CURL_HTTP_VERSION_1_1
+
+Enforce HTTP 1.1 requests.
+
+## CURL_HTTP_VERSION_2_0
+
+Attempt HTTP 2 requests. libcurl falls back to HTTP 1.1 if HTTP 2 cannot be
+negotiated with the server. (Added in 7.33.0)
+
+When libcurl uses HTTP/2 over HTTPS, it does not itself insist on TLS 1.2 or
+higher even though that is required by the specification. A user can add this
+version requirement with CURLOPT_SSLVERSION(3).
+
+The alias *CURL_HTTP_VERSION_2* was added in 7.43.0 to better reflect the
+actual protocol name.
+
+## CURL_HTTP_VERSION_2TLS
+
+Attempt HTTP 2 over TLS (HTTPS) only. libcurl falls back to HTTP 1.1 if HTTP 2
+cannot be negotiated with the HTTPS server. For clear text HTTP servers,
+libcurl uses 1.1. (Added in 7.47.0)
+
+## CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE
+
+Issue non-TLS HTTP requests using HTTP/2 without HTTP/1.1 Upgrade. It requires
+prior knowledge that the server supports HTTP/2 straight away. HTTPS requests
+still do HTTP/2 the standard way with negotiated protocol version in the TLS
+handshake. (Added in 7.49.0)
+
+## CURL_HTTP_VERSION_3
+
+(Added in 7.66.0) This option makes libcurl attempt to use HTTP/3 to the host
+given in the URL, with fallback to earlier HTTP versions if needed.
+
+## CURL_HTTP_VERSION_3ONLY
+
+(Added in 7.88.0) Setting this makes libcurl attempt to use HTTP/3 directly to
+server given in the URL and does not downgrade to earlier HTTP version if the
+server does not support HTTP/3.
+
+# DEFAULT
+
+Since curl 7.62.0: CURL_HTTP_VERSION_2TLS
+
+Before that: CURL_HTTP_VERSION_1_1
+
+# PROTOCOLS
+
+HTTP
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    CURLcode ret;
+    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
+    curl_easy_setopt(curl, CURLOPT_HTTP_VERSION,
+                     (long)CURL_HTTP_VERSION_2TLS);
+    ret = curl_easy_perform(curl);
+    if(ret == CURLE_HTTP_RETURNED_ERROR) {
+      /* an HTTP response error problem */
+    }
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Along with HTTP
+
+# RETURN VALUE
+
+Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not.