Sort out hgroup usage

[annevk]

We need to replace h2 with p and adjust stylesheets while at the same time not upsetting the validator.

cc @sideshowbarker

[domenic]

PR list:

• [x] whatwg.org, including the stylesheets themselves: https://github.com/whatwg/whatwg.org/pull/408
• [x] Bikeshed and thus all non-HTML WHATWG specs: https://github.com/tabatkins/bikeshed-boilerplate/pull/29#pullrequestreview-1168028104
• [x] HTML: https://github.com/whatwg/html/pull/8473
• [x] Participate: https://github.com/whatwg/participate.whatwg.org/pull/311
• [x] Blog: https://github.com/whatwg/blog.whatwg.org/pull/24

[domenic]

All PRs merged, although Bikeshed specs are not yet regenerated.

[annevk]

It seems to have worked, but I did run into style sheet not being updated issues unfortunately.