NewTab/Messaging System dev moved from GitHub to m-c

[Dan Mosedale]

NewTab and Messaging System development is now happening entirely in
mozilla-central, rather than in the now-obsolete
mozilla/activity-stream GitHub repo [0].

As part of this, the in-tree documentation is now exposed through
firefox-source-docs ([1] and [2]).  It updated to be more in line with
current development practices.

If you find any issues, feel free to ping me or file a bug in either
the New Tab or Messaging System component in Bugzilla.  I've already
filed [3] about the fact that the tables aren't currently rendered
correctly in firefox-source-docs.

Dan

[0] https://github.com/mozilla/activity-stream
[1] https://firefox-source-docs.mozilla.org/browser/components/newtab/docs/index.html
[2] https://firefox-source-docs.mozilla.org/browser/components/newtab/content-src/asrouter/docs/index.html
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1619381