📖 Document custom ports feature

[macaptain]

What this PR does / why we need it: Following up on the custom ports feature #876, this adds documentation to let users know about how to use the feature, as suggested in #876 (comment)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #899

Special notes for your reviewer:

Let me know if you'd like me add any further details, or add documentation to any other areas.

TODOs:

• includes documentation

/hold

[k8s-ci-robot]

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[hidekazuna]

As document header and comment suggests, re-run DocToc to update table of contents, please.

[jichenjc]

/ok-to-test

[sbueringer]

Thx for the PR, apart from the ToC /lgtm :)

[macaptain]

As document header and comment suggests, re-run DocToc to update table of contents, please.

That's done. Thanks!

[jichenjc]

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jichenjc, macaptain

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