Add Netlify PR preview deployments

[yarikoptic]

Summary

Add Netlify configuration for automatic PR preview deployments, resolving the long-standing request in #119.

• Production deployment stays on GitHub Pages (docs.dandiarchive.org) — no changes there
• Netlify is used only for PR previews — every PR gets an automatic preview link posted as a comment
• Single file addition: netlify.toml

Setup required (one-time, Netlify dashboard)

Connect Netlify before merging so this PR itself serves as the test:

1. Go to app.netlify.com and sign in with GitHub
2. Click "Add new site" → "Import an existing project" → select GitHub
3. Authorize the Netlify GitHub App for the dandi org (if not already done)
4. Select the dandi/dandi-docs repository
5. Netlify will auto-detect netlify.toml from the PR branch — click "Deploy site"
6. Important: Do NOT add a custom domain (docs.dandiarchive.org) in Netlify — that stays on GitHub Pages
7. Optionally rename the Netlify site to something recognizable (e.g., dandi-docs) in Site configuration → Site details → Change site name

Netlify will immediately pick up this open PR and post a preview link, confirming everything works before merge.

After this, every future PR will automatically get:

• A Netlify build triggered on open/push
• A deploy preview link posted as a PR comment (e.g., https://deploy-preview-42--dandi-docs.netlify.app)
• A GitHub status check showing build pass/fail

Test plan

• netlify.toml build command works: pip install -r requirements.txt && mkdocs build
• Connect repo on Netlify, verify it builds this PR and posts a preview link
• Verify the preview renders correctly

Closes #119

🤖 Generated with Claude Code

[yarikoptic]

may be I do need to merge first... no harm should be so I will proceed

[yarikoptic]

I manually triggered build and we got https://dandi-docs.netlify.app/ ... let's see if PRs rebased now would get their previews