Since Doxygen can't seem to just exclude them via command, they are moved
into a private_api namespace and then excluded via Doxygen config:
* *VTable
* make_dyn_node, ItemTreeNode, etc.
* VersionCheck
Doxygen's markdown support can't quite deal with the Github flavor,
the language reference looks terrible.
So instead, this change switches to using Sphinx,
with two extensions that call Doxygen for us and allow including markdown.
The result is a read-the-docs themed sphinx site that includes search even
in the language reference.