contributing: update style guide for C

[petrasovaa]

This PR adds relevant info from https://trac.osgeo.org/grass/wiki/Submitting/C as mentioned here. I left out some subsections from it, because I was not sure whether they are relevant, for example note about void functions.

I also added missing info about the python addon template and header.

[nilason]

Looks good to me, some minor notes only.

The CI runners will complain with absent void, so I believe there's no need for that info specifically.

Perhaps you can bump the note on ClangFormat version to:
(currently with version 18+), while updating the file.

[nilason]

Great! Always difficult to draw the line on what to include here and still keep it short, if something comes up we can always update.