Docs headings and changelog refactor

[GeoffreyBooth]

This PR is meant to make #5058 easier.
https://rawgit.com/GeoffreyBooth/coffeescript/docs-headings/docs/v2/

• The changelog entries are now split into separate files, and each one is in its own <section> tag like the rest of the docs’ sections; and they use h3 tags like the other sub-sections do. The HTML structure should be consistent now between sections like Breaking Changes and its children and Changelog and its children.
• I made the dates in the changelog headings formatted in the user’s locale by default. So for US English users they appear the same as before, but a U.K. user should see e.g. “13 May 2018” and a French user “13 mai 2018”. @zdenko, if you have any devices with non-US locale settings, please let me know if it works for you 😄The formatting falls back to ISO, e.g. “2018-05-13”.
• I added separate sidebar links for Introduction and Overview, so that the scrolling to those sections works as expected. This also simplifies the structure, as there aren’t any sections with two htmlFor calls (other than for children).

I considered making a sectionFor helper, to slim down body.html, but ultimately figured it would be more hassle than it’s worth because of the complexity of how to handle children. Hopefully this isn’t too hard to parse as is.

[zdenko]

LGTM