Update Storybook and document the @tektoncd/dashboard-* packages

[AlanGreene]

Changes

Add a welcome page to the Storybook providing a brief overview and describing the purpose of each section in the Storybook.

When running in dev mode, include the Carbon storybooks for easier reference. Also include links to commonly used pages in the Carbon documentation.

Add documentation to each of the package folders describing their purpose, any additional required setup, and providing examples of their use.

/kind documentation

Submitter Checklist

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

• Docs included if any changes are user facing
• 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
• Release notes block below has been updated with any user facing changes (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[briangleeson]

/lgtm

[tekton-robot]

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