Move all the wiki pages from MediaWiki format to MarkDown

[icculus]

Move all the wiki pages from MediaWiki format to MarkDown. ghwikipp supports either format, but the headers are already in MarkDown, so we could eliminate conversion in wikiheaders.pl and also be less inhibited in what we do to satisfy that script. Also, MarkDown sucks less to write in.

Originally posted by @icculus in libsdl-org/SDL#3519 (comment)

[icculus]

The bridge cronjob has been disabled for a while, as I'm about to start breaking things to move everything to markdown.

This is just for the headers/wiki bridge, since that can convert the bulk of the wiki in one batch. MediaWiki support will remain, and possible make sense for some pages, plus they would all need to be migrated by hand or some means other than wikiheaders.pl.

[icculus]

Ok, the bridge cronjob is back up and running, and I think most everything is unbusted now and in Markdown format, knock on wood.

The things that aren't bridged are still in mediawiki format, and I'm not going to take the time to manually adjust all of those, so they can live for now.