Docs update: CSI + projected workspaces are beta

[lbernick]

CSI volumes and projected volumes are beta features, and in v1, they can only be used with "enable-api-fields" set to "beta". This commit updates documentation to clarify the status of these features.

/kind documentation

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NONE

[lbernick]

/retest
#6624

[jerop]

/lgtm

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