Proposal: move all documentation to pelias/pelias for hosting on https://pelias.io

We originally have this documentation repo only because Mapzen needed project documentation to be in its own repo so that it could be easily deployed to the Mapzen website. Back then, we didn't really even fully articulate the differences between Mapzen Search (Mapzen's hosted instance of Pelias), and Pelias itself.

At this point all that is history, and we can consider what we could gain by making the main source of Pelias documentation hosted on the Pelias website, https://pelias.io

Here are some:

• One less repo :)
• Having the main source of Pelias documentation on a website allows us to customize how it's displayed
• We would be able to build up the SEO presence of the pelias.io site by linking to it much more heavily
• It's very hard to do "redirects" on GitHub, we have lots of files that basically say "documentation moved to somewhere else" since we can't delete the original file without breaking links

No files or tests are named. Start by reviewing the documentation repository and the Pelias website hosting setup, then define the migration scope, link handling, and deployment changes needed for the documentation to live on pelias.io.