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/Makefile.am

@@ -26,24 +26,21 @@ AUTOMAKE_OPTIONS = foreign no-dependencies
 
 # EXTRA_DIST breaks with $(abs_builddir) so build it using this variable
 # but distribute it (using the relative file name) in the next variable
-man_MANS = $(abs_builddir)/curl.1
+man_MANS = $(abs_builddir)/curl.1 mk-ca-bundle.1
 noinst_man_MANS = curl.1 mk-ca-bundle.1
 dist_man_MANS = curl-config.1
-GENHTMLPAGES = curl.html curl-config.html mk-ca-bundle.html
-PDFPAGES = curl.pdf curl-config.pdf mk-ca-bundle.pdf
-MANDISTPAGES = curl.1.dist curl-config.1.dist
-
-HTMLPAGES = $(GENHTMLPAGES)
+CURLPAGES = curl-config.md mk-ca-bundle.md
 
 # Build targets in this file (.) before cmdline-opts to ensure that
 # the curl.1 rule below runs first
-SUBDIRS = . cmdline-opts
-DIST_SUBDIRS = $(SUBDIRS) examples libcurl
+SUBDIRS = . cmdline-opts libcurl
+DIST_SUBDIRS = $(SUBDIRS) examples
 
-CLEANFILES = $(GENHTMLPAGES) $(PDFPAGES) $(MANDISTPAGES) curl.1
+CLEANFILES = $(man_MANS) curl.1 curl-config.1 mk-ca-bundle.1
+nodist_MANS = $(CLEANFILES)
 
 EXTRA_DIST =                                    \
- $(noinst_man_MANS)                             \
+ $(CURLPAGES)                                   \
  ALTSVC.md                                      \
  BINDINGS.md                                    \
  BUFREF.md                                      \
@@ -59,6 +56,7 @@ EXTRA_DIST =                                    \
  CONNECTION-FILTERS.md                          \
  CONTRIBUTE.md                                  \
  CURL-DISABLE.md                                \
+ CURLDOWN.md                                    \
  DEPRECATE.md                                   \
  DYNBUF.md                                      \
  EARLY-RELEASE.md                               \
@@ -98,9 +96,14 @@ EXTRA_DIST =                                    \
  VULN-DISCLOSURE-POLICY.md                      \
  WEBSOCKET.md
 
-MAN2HTML= roffit $< >$@
+CD2NROFF = $(top_srcdir)/scripts/cd2nroff $< >$@
 
-SUFFIXES = .1 .html .pdf
+CD2 = $(CD2_$(V))
+CD2_0 = @echo "  RENDER " $@;
+CD2_1 =
+CD2_ = $(CD2_0)
+
+SUFFIXES = .1 .md
 
 # $(abs_builddir) is to disable VPATH when searching for this file, which
 # would otherwise find the copy in $(srcdir) which breaks the $(HUGE)
@@ -116,21 +119,10 @@ $(abs_builddir)/curl.1:
 		&& touch -r "$(srcdir)/curl.1" $@; fi
 	cd cmdline-opts && $(MAKE)
 
-html: $(HTMLPAGES)
-	cd libcurl && $(MAKE) html
+.md.1:
+	$(CD2)$(CD2NROFF)
 
-pdf: $(PDFPAGES)
-	cd libcurl && $(MAKE) pdf
-
-.1.html:
-	$(MAN2HTML)
-
-.1.pdf:
-	@(foo=`echo $@ | sed -e 's/\.[0-9]$$//g'`; \
-	groff -Tps -man $< >$$foo.ps; \
-	ps2pdf $$foo.ps $@; \
-	rm $$foo.ps; \
-	echo "converted $< to $@")
+curl-config.1: curl-config.md
 
 distclean:
 	rm -f $(CLEANFILES)