Add v0.26.x and v0.27.x doc

[khrm]

Also marked 0.25.x as LTS. We do four LTS in a year.

Changes

Submitter Checklist

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

• [x] Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• [x] Has Tests included if any functionality added or changed
• [x] Follows the commit message standard
• [x] Meets the Tekton contributor standards (including functionality, content, code)
• [x] 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
• [ ] 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.
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[khrm]

/assign @dibyom

[khrm]

/assign @dibyom Let's merge this.

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