docs: Provide support-matrix-generate, to generate a support matrix in HTML

docs: Provide support-matrix-generate, to generate a support matrix in HTML

This archaeology script:
 - figures out what the current and previous Xen versions were
 - looks for appropriate git branches for them
 - finds SUPPORT.md for each one
 - feeds its findings to parse-support-md

We do not intend to integrate this into docs/Makefile, because it
relies on the git history.  Instead, we will take the rune provided in
the head comment and paste a variant of it into an appropriate cronjob
on xenbits.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Lars Kurth <lars.kurth@citrix.com>
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v3: Provide -D option.