docs: fix issue#6460 remove helm2 section
docs: fix issue#6460 remove helm2 section

1. only helm3 support,deprecated helm2.x docs
2. remove useless helm3 comments
3. update documentation for helm-operator.md

Co-authored-by: Alexander Trost <5390bb27d7067fcf630e3379a1b3cb722c6b2029@googlemail.com>
Signed-off-by: Deshi Xiao <ce6baa22c65be7d48949d3dd3d85a2e1d4063969@gmail.com>

rook/rook,rook/nfs,rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook,rook/nfs