Beefs up variables.md

Adds site.data
Adds site.collections
Adds site.html_pages
Merge branch 'master' of github.com:jekyll/jekyll into beefUpDox
Merge pull request #2567 from austinpray/beefUpDox
Update history to reflect merge of #2567 [ci skip]
Fixed broken link to Permalinks page [ci skip]

The Permalinks link was generating a 404 error due to the link using a capital P in the link path.
Merge pull request #2572 from AJ-Acevedo/master
Update history to reflect merge of #2572 [ci skip]
Add test for document permalink
Solidify document permalink tests
Modify tests a bit
Don't use old `@url` variable

Fix for #2557
Update link to Windows installation guide

I've just released a new version of Run Jekyll on Windows and it's now got its own domain!
Update Maruku dependency to allow use of the latest version.

All Maruku releases post-0.6 follow semver, so they should be backwards-compatible on minor versions. In this case, the only test that needed to change was one that was asserting buggy behavior that was fixed in 0.7.1.
Merge pull request #2578 from juthilo/patch-1
Update history to reflect merge of #2578 [ci skip]
Fix Data Files example around iterative over files in a subfolder of _data.

Addresses @fkostadinov's comment in #2395:
https://github.com/jekyll/jekyll/pull/2395#issuecomment-48151478
Merge pull request #2575 from alfredxing/test-document-permalink
Update history to reflect merge of #2575 [ci skip]
Merge pull request #2576 from bhollis/maruku-update
Update history to reflect merge of #2576 [ci skip]