Bug 1441019 - Add overviews and links to new API docs. r=whimboo

Adds some links to the frontpage and an index that lists all the
internals.

MozReview-Commit-ID: HHk1YCtUMKU

--HG--
extra : rebase_source : 0c281d7f113748509a4d968b947bd460bfe78be1

testing/marionette/doc/index.rst

@@ -51,6 +51,7 @@ For developers
    Debugging.md
    PythonTests.md
    SeleniumAtoms.md
+   internals/index
 
 .. _Contributing to Marionette: https://searchfox.org/mozilla-central/source/testing/marionette/CONTRIBUTING.md
 

testing/marionette/doc/internals/index.rst

@@ -0,0 +1,11 @@
+=========
+Internals
+=========
+
+This is an overview of all documented internals in the Marionette server:
+
+.. toctree::
+  :glob:
+  :maxdepth: 1
+
+  *