Update documentation to reflect stability levels and deprecations

[lbernick]

This commit updates docs to reflect the fact that PipelineResources have been removed and that remote resolution is a beta feature.

/kind documentation

Submitter Checklist

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

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• n/a Has Tests included if any functionality added or changed
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• 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). See some examples of good release notes.
• 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

[lbernick]

/assign @abayer -- can you review the bits about remote resolution being a beta feature?

[dibyom]

/assign

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel, dibyom

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[chitrangpatel]

/lgtm

[chitrangpatel]

/lgtm