update configure-kn.md to include channel-type-mappings and add comments to sample yaml

[garg-dhruv]

Proposed Changes

• Add 'channel-type-mappings' customisation option to Customizing kn docs.
  This feature already existed and was documented as part of the github docs for kn-client, but was missing from the official website. Please take a look at this comment.
• Also, replaced the config yaml with a better-commented version of the same, which already existed as part of the github docs for kn-client.

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[knative-prow]

Welcome @garg-dhruv! It looks like this is your first PR to knative/docs 🎉

[garg-dhruv]

/assign @dsimansk

[skonto]

cc @rhuss

[rhuss]

looks good to me, maybe update the example version as @dsimansk suggest. Then we are good to merge.

[dsimansk]

/approve
/lgtm
/cherry-pick release-1.11

[knative-prow-robot]

@dsimansk: once the present PR merges, I will cherry-pick it on top of release-1.11 in a new PR and assign it to you.

In response to this:

/approve
/lgtm
/cherry-pick release-1.11

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[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, garg-dhruv

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[knative-prow-robot]

@dsimansk: new pull request created: #5646

In response to this:

/approve
/lgtm
/cherry-pick release-1.11

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/test-infra repository.