📖 Add release 1.7 book link

[chandankumar4]

What this PR does / why we need it:
Document that the default documented version is now release-1.8 and provide backlink to release-1.7

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

Part of #10472

/area release

[k8s-ci-robot]

Hi @chandankumar4. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[killianmuldoon]

/ok-to-test

/lgtm

[fabriziopandini]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 84ab8699279d97d88bae0c25f7e9560253b0d10a

[sbueringer]

/lgtm
/approve

/cherry-pick release-1.8

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.8 in a new PR and assign it to you.

In response to this:

/lgtm
/approve

/cherry-pick release-1.8

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #10966

In response to this:

/lgtm
/approve

/cherry-pick release-1.8

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.