doc: add snapshot feature doc

[umagnus]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

/kind documentation

What this PR does / why we need it:

doc: add snapshot feature doc

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

none

[k8s-ci-robot]

Hi @umagnus. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[andyzhangx]

/ok-to-test
/lgtm

[andyzhangx]

could you keep the path of snapshotclass-nfs.yaml, otherwise e2e test would fail:

+ sed -i s/nfs.csi.k8s.io/test.csi.k8s.io/g deploy/example/snapshotclass-nfs.yaml
sed: can't read deploy/example/snapshotclass-nfs.yaml: No such file or directory
make: *** [Makefile:159: e2e-test] Error 2

[umagnus]

deploy/example/snapshotclass-nfs.yaml

fix the path in external-e2e run.sh

[andyzhangx]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, umagnus

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