Fix helm 3 crds installation + add some docs

[PERES-Richard]

Q	A
Bug fix?	[x]

What's in this PR?

Why?

The actual installation with helm 3 use a crd-install hook wich is depreciated in helm 3. With this fix you can now install Casskop (CRD + operator) with just 1 line of helm.

Additional context

It simply consist of copying CRDs from /deploy/crds to helm folders of Casskop & Multi Casskop.

Checklist

• [x] Implementation tested
• [x] Logging code meets the guideline
• [x] User guide and development docs updated (if needed)
• [ ] Append changelog

To Do

• [x] Test & changelog
• [ ] Deploy new artifact

[cscetbon]

@PERES-Richard are you sure helm2 still works with those changes ? #133 seems to say it doesn't. And if doesn't I think we can drop support for helm 2.