So that it's easy for anyone to jump to the new docs that we are
currently working on, and intend to replace the existing docs with.
While I would have preferred to link to the local dev page, it's still
stuck in the PR state, currently blocked on another PR:
https://github.com/dagger/dagger/pull/1586
Also added a link to the pre-Europa docs, so that it's easy to go back.
While at it, drop "Sidebar" from the name of sidebars, and replace
tutorial with a more descriptive name.
Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>
