libgccjit documentation now available on gcc.gnu.org

[David Malcolm]

The documentation for libgccjit is now visible on the GCC website in
HTML form at:
  https://gcc.gnu.org/onlinedocs/jit/
(and is being built nightly).

Thanks to everyone who helped make this happen (Gerald Pfeifer, Joseph
Myers, Frank Ch. Eigler, Jeff Law, iirc).

The "Show Source" links on each page are currently 404s, but that should
be resolved shortly.

Dave