Better organize existing Markdown documentation

[nylen]

We have multiple Markdown documentation files within the Gutenberg repository that are currently not so easy to find: https://github.com/WordPress/gutenberg/search?l=Markdown

Potential ways to improve this:

• Build a table of contents into the main readme, or other ways of pointing to these documents.
• Integrate more of our existing documentation into http://gutenberg-devdoc.surge.sh/ (and prominently link to this site from our main readme).

[youknowriad]

I've already included most of them in http://gutenberg-devdoc.surge.sh/ but we should definitely link to the site from the README 👍

We should also add more documentation to our components, we have only one component documented right now.

[mtias]

This will be linked from a canonical page under .org website.