Update Book Project and Books API docs

[knjk04]

Find a way to update the Book Project and Books API contributing and code of conduct files with the date and time (DD/MM/YYYY HH:MM) the last time the file was changed on the docs site.

Why?
GitHub has a new feature that tells contributors when opening an issue that the file was recently changed. However, as we are now hosting those files on this docs site, those files with links may seldom change, while the actual contributing and code of conduct files on the site may. Adding a timestamp can let a contributor know when the last time the file was changed, which will allow them to re-review the files.