docs: add discuss-first PR classes and tutorial-data upload path

[bobleesj]

Summary

• Split pull requests into discuss-first (new widgets, shared helpers, public API / tutorial-data contract) and incremental in-widget fixes.
• Point tutorial file uploads at the public bobleesj/quantem-data dataset card (Write token, python -m check_meta, hf upload --create-pr). GitHub PRs here are only for a named quantem.widget.datasets loader.
• Add a copyable prompt on Agent-assisted development so a coding agent can open that Hugging Face Community pull request after the human sets the token and attests permission.

Test plan

• Read docs/maintainer/pull-requests.md and docs/tutorials/contribute_data.md as a new contributor
• Confirm docs/agent-prompts.md has the Hugging Face tutorial-data prompt
• Confirm Example Data no longer shows quantem.widget.io.upload for tutorial fixtures
• After the matching Hugging Face dataset PR merges, follow the card token and python -m check_meta steps

[github-actions]

• Delete bobleesj/quantem.widget:aug-19-pr-and-tutorial-data from its remote if you own it, it is not long-lived, and no other PR uses it.
• Remove the dedicated worktree and local aug-19-pr-and-tutorial-data branch after confirming the worktree is clean.
• Fetch electronmicroscopy/quantem.widget and update local main before starting new work.

[github-actions]

This PR is 1 commit behind electronmicroscopy/quantem.widget:main.

Fetch that remote and update local main before you branch, or rebase this branch onto it before review.
A warning is enough. The PR can stay open while main moves.