[css-transitions] Convert CSS Transitions spec to bikeshed preprocessor.

Known regressions from this conversion:
- extra sentence in abstract that I don't like
- custom sentence in SotD is lost
- putting 'transition-property' in a link yields nested link (bad markup)
- breaks some within-IDL links

Note that this also adds a conformance section not previously present,
and also an IDL Index and (when issues present in document) Issues Index.

Author: dbaron