build: generate new HTML documentation

This revamped doc target converts and installs the remaining bits of
documentation mentioned in the sysusers manpages.

Markdown documentation is converted using cmark-gfm
if available, falling back to cmark or the original markdown.
When cmark-gfm is not found, rendering of GitHub Flavoured Markdown
features like tables is broken, but everything else is rendered fine.

The generated HTML pages don't look like the ones seen on systemd.io,
but I don't think it matters much; the important information is in the
text, not the graphics.

Addresses my previous comment,
#1 (comment)

Author: Tachi107

man/Makefile

@@ -1,6 +1,6 @@
 manfiles5 = sysusers.d.5
 manfiles8 = systemd-sysusers.8 systemd-sysusers.service.8
-docfiles = sysusers.d.html systemd-sysusers.html
+docfiles = UIDS-GIDS.html USER_NAMES.html
 
 xsltargs = --nonet \
 	--xinclude \
@@ -21,8 +21,15 @@ man:
 	xsltproc $(xsltargs) custom-man.xsl sysusers.d.xml
 
 doc:
-	xsltproc $(xsltargs) -o systemd-sysusers.html custom-html.xsl systemd-sysusers.xml
-	xsltproc $(xsltargs) -o sysusers.d.html custom-html.xsl sysusers.d.xml
+	if command -v cmark-gfm; then \
+		md='cmark-gfm --extension table'; \
+	elif command -v cmark; then \
+		md='cmark'; \
+	else \
+		md='markdown'; \
+	fi; \
+	$$md UIDS-GIDS.md > UIDS-GIDS.html; \
+	$$md USER_NAMES.md > USER_NAMES.html
 
 # Delete files because wget can't overwrite them
 download-docs:
@@ -39,7 +46,9 @@ download-docs:
 		'https://github.com/systemd/systemd/raw/main/man/custom-man.xsl' \
 		'https://github.com/systemd/systemd/raw/main/man/standard-options.xml' \
 		'https://github.com/systemd/systemd/raw/main/man/systemd-sysusers.xml' \
-		'https://github.com/systemd/systemd/raw/main/man/sysusers.d.xml'
+		'https://github.com/systemd/systemd/raw/main/man/sysusers.d.xml' \
+		'https://github.com/systemd/systemd/raw/main/docs/UIDS-GIDS.md' \
+		'https://github.com/systemd/systemd/raw/main/docs/USER_NAMES.md'
 
 clean:
 	rm -f $(manfiles5)