docs: add strict-mode and property-descriptors concept pages with tests

- Add comprehensive strict-mode concept page (27 tests passing)
  - Covers what strict mode does, how to enable it, and key restrictions
  - Includes SEO optimization with internal links and meta description

- Add comprehensive property-descriptors concept page (50 tests passing)
  - Covers writable, enumerable, configurable flags
  - Object.defineProperty(), accessor descriptors, object-level protections
  - Fixed broken external links (freeCodeCamp, DigitalOcean)

Both pages reviewed with write-concept, fact-check, and seo-review skills.