Moved namespace docs to a single file.

As suggested in PR 2802 (https://github.com/mono/MonoGame/pull/2802), moved namespace documentation into its own file. Included this file in the project definition file.

Also, (possibly) unrelated: it looks like the documentation page on the website is currently empty (http://www.monogame.net/documentation/). Not sure if this is related to the previous PRs with documentation changes.