This PR adds a documentation page that lists all available settings. The
page is automatically generated.
Having the settings listed in the online documentation makes it easier
to search for users. It also has the advantage of being indexed by
search engines.
To generate the docs, we first generate JSON out of the TableGen
definitions with `-dump-json`.
Once all properties are available as JSON, a Markdown file with the
merged documentation (`settings.md`) is generated. I chose Markdown over
RST, because some descriptions already use backticks, which would become
references in RST.
Currently, enum names/descriptions are not visible, because they're
defined in C++. In the future, these could be moved to TableGen as well.
ce8251e60a