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

@@ -0,0 +1,70 @@
+# Configuration file for the Sphinx documentation builder.
+#
+# This file only contains a selection of the most common options. For a full
+# list see the documentation:
+# https://www.sphinx-doc.org/en/master/usage/configuration.html
+
+# -- Path setup --------------------------------------------------------------
+
+# If extensions (or modules to document with autodoc) are in another directory,
+# add these directories to sys.path here. If the directory is relative to the
+# documentation root, use os.path.abspath to make it absolute, like shown here.
+#
+# import os
+# import sys
+# sys.path.insert(0, os.path.abspath('.'))
+
+
+# -- Project information -----------------------------------------------------
+
+project = 'SixtyFPS C++'
+copyright = '2020, info@sixtyfps.io'
+author = 'info@sixtyfps.io'
+
+# The full version, including alpha/beta/rc tags
+release = '0.0.1'
+
+
+# -- General configuration ---------------------------------------------------
+
+# Add any Sphinx extension module names here, as strings. They can be
+# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
+# ones.
+extensions = ["breathe", "recommonmark", "exhale"]
+
+breathe_projects = {
+        "SixtyFPS": "./docs/xml"
+}
+breathe_default_project = "SixtyFPS"
+
+exhale_args = {
+        "containmentFolder": "./api",
+        "rootFileName": "library_root.rst",
+        "rootFileTitle": "SixtyFPS CPP Reference Documentation",
+        "doxygenStripFromPath": "..",
+        "createTreeView": True,
+        "exhaleExecutesDoxygen": True,
+        "exhaleDoxygenStdin": '''INPUT = ../../api/sixtyfps-cpp/include
+EXCLUDE_SYMBOLS = sixtyfps::cbindgen_private* sixtyfps::private_api*'''
+}
+
+# Add any paths that contain templates here, relative to this directory.
+templates_path = ['_templates']
+
+# List of patterns, relative to source directory, that match files and
+# directories to ignore when looking for source files.
+# This pattern also affects html_static_path and html_extra_path.
+exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
+
+
+# -- Options for HTML output -------------------------------------------------
+
+# The theme to use for HTML and HTML Help pages.  See the documentation for
+# a list of builtin themes.
+#
+html_theme = 'sphinx_rtd_theme'
+
+# Add any paths that contain custom static files (such as style sheets) here,
+# relative to this directory. They are copied after the builtin static files,
+# so a file named "default.css" will overwrite the builtin "default.css".
+#html_static_path = ['_static']