update CEL filtering documentation

[avinal]

Changes

• update the filters list with currently supported filtering expressions
• add description and example for most used filters
• reformat markdown, fix tables and lint warnings

Submitter Checklist

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

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

Release Notes

NONE

[avinal]

/kind documentation

[avinal]

cc: @khrm @enarha @sayan-biswas @alan-ghelardi

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adambkaplan, khrm

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/results/blob/main/OWNERS)~~ [adambkaplan,khrm] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment