Make C++ documentation generation an xtask

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).

api/sixtyfps-cpp/docs/conf.py

@@ -44,7 +44,7 @@ exhale_args = {
         "doxygenStripFromPath": "..",
         "createTreeView": True,
         "exhaleExecutesDoxygen": True,
-        "exhaleDoxygenStdin": '''INPUT = @CMAKE_CURRENT_SOURCE_DIR@/../include
+        "exhaleDoxygenStdin": '''INPUT = ../../api/sixtyfps-cpp/include
 EXCLUDE_SYMBOLS = sixtyfps::cbindgen_private* sixtyfps::private_api*'''
 }