Set up mkdocs-material documentation site

[talmo]

Summary

Set up a documentation site using mkdocs-material, served at {repo-root}/docs and deployed alongside the app.

Requirements

• mkdocs.yml config at repo root
• docs/ directory with markdown content (currently empty after cleanup)
• GitHub Actions workflow to build and deploy docs
• Docs accessible at a docs path or subdomain (TBD)

Content to include

• Getting started / installation guide
• User guide (labeling workflows, keyboard shortcuts)
• Architecture overview (can be regenerated from README)
• Development setup and contributing guide
• API/data format documentation (SLP files, sleap-io.js)