docs: add usage and developer documentation

[jmagly]

Summary

Adds a documentation base for users and developers, plus Pagenary docsite publishing support:

• docs/GETTING_STARTED.md for install, configuration, first safe run, CLI basics, updates, and troubleshooting
• docs/README.md as the docs landing page
• docs/DEVELOPER_GUIDE.md for source layout, development commands, SDK usage, extension points, and docsite workflow
• docs/API_REFERENCE.md with the current local HTTP route surface grouped by workflow
• docs/MCP_GUIDE.md for the stdio MCP server and security_recon
• docsite/t3mp3st-docs/ Pagenary tenant source with config, manifest, generated content pages, and page metadata
• root tenants.json registry for Pagenary publishing
• npm scripts for docsite generation, build, verification, and local preview
• README links to the new usage and developer docs

Closes #102

Pagenary workflow

• npm run docs:sync regenerates the Pagenary tenant source from canonical Markdown docs
• npm run docs:build builds the static Pagenary docsite
• npm run docs:check verifies generated publishing artifacts and indexed page coverage
• npm run docs:serve builds and serves the docsite locally at /t3mp3st-docs/
• PAGENARY_SITE_URL can override the canonical production URL during docsite generation

Validation

• git diff --check
• npm install
• npm run docs:check
• npm run build
• npm test
• HTTP smoke checks against local Pagenary preview at http://127.0.0.1:5173/t3mp3st-docs/
• route-reference check: all 116 Express routes in src/server.ts are referenced in docs/API_REFERENCE.md

Note: Playwright browser inspection could not run in this environment because the local Playwright browser extension was unavailable, so docsite preview verification used Pagenary build/check output plus HTTP smoke checks.