Document the CCE example input file

[nilsdeppe]

Proposed changes

This is to help people like me who don't know what anything means actually be able to change things in a meaningful way.

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

[nilsdeppe]

I've rebased and squashed changes. Thanks for the reviews!

[nilsdeppe]

Whoops! Done, thanks!