(DOC-909) Remove HTML comments from markdown source files

Partly due to inconsistent behavior across markdown renderers, and partly due to
the comments often being stale references to work we never got around to doing,
we're gonna declare bankruptcy and get rid of them all. (Except a few useful
ones in the platforms_* include files.)
(DOC-909) Remove several Liquid comments

These didn't seem to be helping us much, so I axed them too.