Add developer documentation on k8s controllers

[lbernick]

Changes

This commit adds details for new contributors on how Tekton controllers and webhooks are implemented, with links to helpful references.

/kind documentation

Closes #5126.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• n/a Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• n/a Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• n/a Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[dibyom]

This is really nice!

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dibyom

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[abayer]

/lgtm

[lbernick]

/retest