Remove migrated content

[Simran-B]

3.10 and above now at https://github.com/arangodb/docs-hugo via https://github.com/arangodb/docs-hugo/pull/199

Uses a link to https://docs.arangodb.com/3.11/get-started/set-up-a-cloud-instance/ in the ArangoGraph start page - if we move or rename this file, we need to update the link!

Upstreams:

• 3.10: https://github.com/arangodb/arangodb/pull/19764
• 3.11: https://github.com/arangodb/arangodb/pull/19763
• 3.12: https://github.com/arangodb/arangodb/pull/19758