Tear it out of the CMakeLists.txt and instead run it via
cargo xtask cppdocs
This allows the build_and_test step in the CI to only run cmake for the
library/header related bits and the docs_and_demos step to only generate
docs and not require a full host build of the library (as cargo xtask
cmake would otherwise do).
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.
