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/CURLDOWN.md

@@ -0,0 +1,125 @@
+# curldown
+
+A markdown-like syntax for libcurl man pages.
+
+## Purpose
+
+A text format for writing libcurl documentation in the shape of man pages.
+
+Make it easier for users to contribute and write documentation. A format that
+is easier on the eye in its source format.
+
+Make it harder to do syntactical mistakes.
+
+Use a format that allows creating man pages that end up looking exactly like
+the man pages did when we wrote them in nroff format.
+
+Take advantage of the fact that people these days are accustomed to markdown
+by using a markdown-like syntax.
+
+This allows us to fix issues in the nroff format easier since now we generate
+them. For example: escaping minus to prevent them from being turned into
+Unicode by man.
+
+Generate nroff output that looks (next to) *identical* to the previous files,
+so that the look, existing test cases, HTML conversions, existing
+infrastructure etc remain mostly intact.
+
+Contains meta-data in a structured way to allow better output (for example the
+see also information) and general awareness of what the file is about.
+
+## File extension
+
+Since curldown looks similar to markdown, we use `.md` extensions on the
+files.
+
+## Conversion
+
+Convert **from curldown to nroff** with `cd2nroff`. Generates nroff man pages.
+
+Convert **from nroff to curldown** with `nroff2cd`. This is only meant to be
+used for the initial conversion to curldown and should ideally never be needed
+again.
+
+Convert, check or clean up an existing curldown to nicer, better, cleaner
+curldown with **cd2cd**.
+
+Mass-convert all curldown files to nroff in specified directories with
+`cdall`:
+
+    cdall [dir1] [dir2] [dir3] ..
+
+## Known issues
+
+The `cd2nroff` tool does not yet handle *italics* or **bold** where the start
+and the end markers are used on separate lines.
+
+The `nroff2cd` tool generates code style quotes for all `.fi` sections since
+the nroff format does not carry a distinction.
+
+# Format
+
+Each curldown starts with a header with meta-data:
+
+    ---
+    c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+    SPDX-License-Identifier: curl
+    Title: CURLOPT_AWS_SIGV4
+    Section: 3
+    Source: libcurl
+    See-also:
+      - CURLOPT_HEADEROPT (3)
+      - CURLOPT_HTTPAUTH (3)
+    ---
+
+All curldown files *must* have all the headers present and at least one
+`See-also:` entry specified.
+
+Following the header in the file, is the manual page using markdown-like
+syntax:
+
+~~~
+    # NAME
+    a page - this is a page descriving something
+
+    # SYNOPSIS
+    ~~~c
+    #include <curl/curl.h>
+
+    CURLcode curl_easy_setopt(CURL *handle, CURLOPT_AWS_SIGV4, char *param);
+    ~~~
+~~~
+
+Quoted source code should start with `~~~c` and end with `~~~` while regular
+quotes can start with `~~~` or just be indented with 4 spaces.
+
+Headers at top-level `#` get converted to `.SH`.
+
+`nroff2cd` supports the `##` next level header which gets converted to `.IP`.
+
+Write bold words or phrases within `**` like:
+
+    This is a **bold** word.
+
+Write italics like:
+
+    This is *italics*.
+
+Due to how man pages don't support backticks especially formatted, such
+occurrences in the source will instead just use italics in the generated
+output:
+
+    This `word` appears in italics.
+
+When generating the nroff output, the tooling will remove superfluous newlines,
+meaning they can be used freely in the source file to make the text more
+readable.
+
+All mentioned curl symbols that have their own man pages, like
+`curl_easy_perform(3)` will automatically be rendered using italics in the
+output without having to enclose it with asterisks. This helps ensuring that
+they get converted to links properly later in the HTML version on the website,
+as converted with `roffit`. This makes the curldown text easier to read even
+when mentioning many curl symbols.
+
+This auto-linking works for patterns matching `(lib|)curl[^ ]*(3)`.