Add a directory housing shared files used in the Kedro docs

[tynandebold]

These files aren't checked into source control anywhere. This is an initial push to get them in so we don't lose access to them.

Perhaps they should live in a more appropriate place? Adding them to this repo made the most sense to me, short of creating a new repo just for these files, which seemed like overkill.

[astrojuanlu]

Closing in favour of https://github.com/kedro-org/kedro-sphinx-theme :)