[CDRIVER-3034] New Installation/Getting-Started Documentation Pages (#1371)

* New documentation with new theming
* Prettier structure and output:
- Use subdirectories for organization.
- Import sphinx-design, which gives us useful components for richer HTML output.

* Migrate most of the installation docs away:
- New install docs and "supported platforms" have been refactored and
  replaced.
- The old "installing" page has been detached from the ToC and is now
  a placeholder referencing the new documentation pages. This will allow
  existing links in the wild to continue to work, while making the page
  itself invisible in the ToC tree.
* Create a howto on selecting an installation location
* Clean up and clarify platform support
* No dep on sphinxcontrib-moderncmakedomain:

Add a very minimal Sphinx extension sufficient to resolve references to
the CMake Sphinx object inventory. We don't need any more features for
now.

* Fix up package builds to install sphinx-design deps
* Vendor external inventories semi-automatically.

To support fully-offline builds of the documentation, vendor external
documentation inventories for intersphinx. These can be updated by
passing '-D update_external_inventories=1' to the sphinx-build command