docs: remove contributor guides from storybook

[graynorton]

Description

This PR removes contributor-facing guides from 2nd-gen Storybook. This information is now in CONTRIBUTOR-DOCS.

It also adds a placeholder "Welcome" guide.

Motivation and context

Keep Storybook focused on customer docs, putting all contributor-facing docs in CONTRIBUTOR-DOCS.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 11cb229

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

📚 Branch Preview

• Documentation Site
• Storybook

🔍 Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-5846

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.

[github-actions]

Tachometer results

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