📖 Restructure ClusterClass documentation

[sbueringer]

Signed-off-by: Stefan Büringer buringerst@vmware.com

What this PR does / why we need it:
The goal of this PR is to create a structure for our ClusterClass documentation so we can added more documentation incrementally. The idea is also to essentially just "move up" the ClusterClass tree in the sidebar once the feature is not experimental anymore.

Changes:

• Dropped ClusterClass quickstart in favor of the "main" Quick start
• Moved ClusterClass upgrades into the operations guide
• Added a new How to write a ClusterClass page (which will be written in a follow-up PR)
• Added a list with useful links to the "starting page" of the experimental ClusterClass feature

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[fabriziopandini]

Nit:
should we move everything related to ClusterClass under ./tasks/experimental-features/cluster-class/, so we can drop cluster class prefix from the files (cluster-classes.md could be renamed in overview.md or something similar)

Also, wondering if we have a clear definition of what is an "how-to" and what are "operations". A thing that we can do is to just drop the distinction and have focused pages on each topic

[sbueringer]

@fabriziopandini I moved the ClusterClass docs into a sub-folder and restructured it as suggested. Please take another look

[stmcginnis]

/lgtm

A couple very minor things pointed out, but nothing that I think is of real concern.

[sbueringer]

@stmcginnis Thank you! Should be fixed

[vincepri]

/approve
/lgtm

[vincepri]

Great work here, thank you!

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