rustdoc: Add stability and source links to the methods. Fixes #12932.

this change made most functions that render the methods to require
the render `Context`, so they are now made into `Context` methods.

Conflicts:
	src/librustdoc/html/markdown.rs
	src/librustdoc/html/render.rs
	src/librustdoc/html/static/main.css

Conflicts:
	src/librustdoc/html/render.rs