Documentation: Initial pandoc conversion

[yadij]

Add pandoc tools dependency to Squid.

Initial documentation file conversion to Markdown
see https://github.github.com/gfm/ for syntax.

Also, updated and polished the squid.8 content to
comply better with manual practices and Squid
current features / behaviours.

[kinkie]

I would support GFM as a dialect; it has powerful backing and should be strict enough that we wouldn't end up in the situation we had with the wiki. I expect that all that's needed to adopt it is to change the argument to pandoc, so no need to redo translation work.

@yadij , can you agree to this?

[kinkie]

Currently, the biggest missing/unknown piece of information for me here is whether gfm supports metadata blocks

gfm supports - in fact, requires to be practically used - metadata in the form of Front Matter

[rousskov]

Reviewer insists that we select a final and permanent Markdown syntax up-front and only use that one syntax forever, in all places Squid can or may now or in future utilize Markdown.

Nobody insists on (or even suggests) that.

[kinkie]

I tested this; it works. Thanks!