Improve handling of unknown elements

When I implemented the docfx tool I skipped handling of most elements in a ``. There are about 300 of them:

https://github.com/doxygen/doxygen/blob/2717489cb90ffd338217b6ce6d2fcb38f25495e6/src/htmlentity.cpp#L40

And we only use about a dozen:

https://github.com/googleapis/google-cloud-cpp/blob/e6140c8884293ada510dd64288d4077afaa69c3a/docfx/doxygen2markdown.cc#L435-L456

The `docfx` tool intentionally crashes when it finds an element it does not know how to handle. That way we can update the code when needed. But maybe we should proactively add elements that we think may appear in the future and are safe to handle.