Do a basic accessibility pass over the docs #1239

Issue:
I don't have any disability, came to the docs (https://rethinkdb.com/docs/install/windows/) and was finding it slightly hard to read. Ran a quick checker tool (WAVE plugin for Chrome), and as I suspected, loads of very low contrast text. This is impeding readability for someone with 20/20 vision, so I assume makes it impossible for people with any visual impairment.

Suggestion:
Consider doing a basic accessibility check. There are tools like the WAVE plugin which allow very quick checks of a few basics, or https://accessibilityinsights.io/ which offers more detailed reports. I ran its fast-pass assessment, and it looks like a few simple fixes (contrast, add a lang attribute to the html tag, and ensure images have alt text) would make a big difference.