Limit ControllerRing name length, simplify shard/drain label keys, simplify webhook name

[timebertt]

What this PR does / why we need it:

This PR introduces a limit on the ControllerRing length, so that we can simplify the shard and drain labels.
On the way, we simplify the webhook name in the same way.

⚠️ Please consult the updated documentation and examples for how to adapt to this breaking change.

• The ControllerRing name must not be longer than 63 characters.
• The shard and drain label keys have been simplified to {shard,drain}.alpha.sharding.timebertt.dev/<controllerring-name>.
• The MutatingWebhookConfiguration name has been simplified to controllerring-<controllerring-name>.
• There is no migration path available.

Which issue(s) this PR fixes:
Fixes #431

Special notes for your reviewer: