These should have been removed in #330.

For both Ubuntu 24.04 and RockyLinux 9. Both files are just copies of
the respective CUDA 12 Dockerfiles with `12.8.1` changed to `13.0.2`.

Also include them in the container image jobs on CI.

It annoys me to have stuff split between the README and the Guide. This
commit moves most of the README contents into the Guide's introduction,
(which was all but empty anyway). I also did some line editing of the
moved text.

Other minor structural changes:
- Rename the first chapter file from `README.md` (not to be confused
  with the top-level README) to `introduction.md`, to match the chaper
  name.
- Move "Supported Features" and the FAQ to unnumbered chapters at the
  end -- making them quasi-appendices -- because they're supplementary.
  This means the "Getting Started" section now follows immediately on
  from the introduction, which is good.

Something I learned recently.

This showed up in #220 and #327.

This file currently uses a mix of 2-space and 4-space indents in
examples.

Currently they don't show up. They need to be within the `src/`
directory to get included in the generated output.