Some changes to sections and references.

Including sections with no notables changes by putting a single
"- Nothing." entry was a bad idea because for every single release this
created a ton of noise. My only gripe with this decision is that users
now have to assume that — for instance — the "Deprecated" section
was intentionally left out because it contained nothing, while it's quite
possible that they maintainers may not have deemed deprecations
worthy of a change log entry. That's a problem because it makes
expectations hard to manage.

Another change I've made is that I now refer to a "change log" (lower case, two words) instead of a "CHANGELOG" (uppercase, one word)
because I've released that the file is named "CHANGELOG" but the
purpose of the file matters more than its name. Moreover the previous
spelling made for awkward sentences and style.
Use iframe pointing to CHANGELOG instead of image.

The static image was a stop-gap to give people a visual rendition of a
raw CHANGELOG file. I find this iframe solution much more satisfying
because it allows people to scroll around the CHANGELOG file.

It's also a LOT easier to maintain since... I don't have to update the
image ever again. Wee!
Add a clear explanation about the CHANGELOG name.

I used to refer exclusively to a "CHANGELOG" instead of a much more
elegant "change log", so I expanded the section about naming to
explain the difference and add a bit of historical information about
why these files are so shouty.
Use GitHub-Pages (Jekyll) for auto-build
Remove ruby files from manual build
Ignore `_site/`
Merge branch 'develop' into gh-pages

* develop:
  Ignore `_site/`
  Remove ruby files from manual build
  Use GitHub-Pages (Jekyll) for auto-build