Fix 'Edit this page' and 'Create an issue' links on the website

[AlanGreene]

Changes

tektoncd/website#65

Add a cascading page property to the frontmatter of docs/README.md
that will apply to all child pages displayed in the pipeline section
of the docs on the website.

This ensures the 'Edit this page' and 'Create an issue' links in
the sidebar point to the pipeline repo instead of the website repo.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task to build and release this image.

Reviewer Notes

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Release Notes

[tekton-robot]

This PR cannot be merged: expecting exactly one kind/ label

Available kind/ labels are:

kind/bug: Categorizes issue or PR as related to a bug.
kind/flake: Categorizes issue or PR as related to a flakey test
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[tekton-robot]

This PR cannot be merged: expecting exactly one kind/ label

Available kind/ labels are:

kind/bug: Categorizes issue or PR as related to a bug.
kind/flake: Categorizes issue or PR as related to a flakey test
kind/cleanup: Categorizes issue or PR as related to cleaning up code, process, or technical debt.
kind/design: Categorizes issue or PR as related to design.
kind/documentation: Categorizes issue or PR as related to documentation.
kind/feature: Categorizes issue or PR as related to a new feature.
kind/misc: Categorizes issue or PR as a miscellaneuous one.

[AlanGreene]

/kind documentation

Maybe this should be misc? Documentation seems a close enough match here, happy to change it though.

[tekton-robot]

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[dibyom]

/lgtm