docs: release notes for 0.88

[nishantmonu51]

Adds release notes for Rill 0.88, covering changes between release-0.87 and release-0.88.

• Headline sections: canvas tab groups, canvas PDF export, tags for all resources, URI dimensions as links, cloud editing improvements, personal canvas dashboards (beta), and Starter/Growth plans
• Categorized bug fixes and improvements, excluding items already announced in the 0.87 notes and including a few fixes from the 0.87 cycle that were never announced

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!