Add Mintlify documentation and simplify README (#24)

Add Mintlify documentation and simplify README (#24)

* Add Mintlify documentation and simplify README

Documentation:
- Add complete Mintlify docs with introduction, quickstart, integrations
- Add concept guides: mask mode, route mode, PII detection, secrets detection
- Add API reference: chat completions, models, status, dashboard API
- Add configuration guides: overview, providers, PII, secrets, logging
- Include dashboard screenshot and branding assets

README:
- Simplify structure, move detailed docs to Mintlify
- Add centered badges and navigation links
- Add "What is PasteGuard?" section explaining problem/solution
- Update example to Dr. Sarah Chen (consistent across all docs)
- Reorder integrations (OpenAI SDK, LangChain, LlamaIndex first)
- Move Presidio attribution inline with PII section
- Add Tech Stack section

Code:
- Update description to "Privacy proxy for LLMs" in package.json,
  startup banner, and /info endpoint

Closes #21

* docs: fix secrets default to block (matches current code)