Consistent documentation

[kvark]

We have 3 sources of documentation right now:

1. in-code
2. in tree: /webrender/doc
3. Github Wiki

It would be great to be consistent as well as bring some structure to the docs.

[nical]

I think that it's fine to have the in-code and in-tree documentation side by side (the latter being good for long high-level explanations), I would really like the in-tree version to be included in the cargo generated doc(*). The wiki doc could move in the in-tree version (main change would be that you would need to pass reviews to change the doc).

Or we could move the in-tree doc to the wiki until cargo doc gains support for including markdown files.

*: Unfortunately it looks like this isn't possible yet without extensions, see rust-lang/cargo#739