Add the JSDoc Markdown plugin so that inline code snippets look their
very best. This plugin is currently used by the Popups repo and adding
it here also improves JSDoc config consistency.
Change-Id: Ib0094bed44268a1002d19ccb1a2495560331e09d
Given the level of documentation in this repo is extremely
low now seems a good as time as any.
The resulting documentation is a vast improvement on before
grouping functions to an owner and introducing module names.
We can iterate on the documentation over time.
Bug: T138401
Change-Id: I9e787d40931d3d72a7037ce07d3ab0db8e72d825