Fix spatial docs: real Census data, chart alignment, pin mkdocs <2

- Replace fake/random dot density data with real ACS race/ethnicity data
  for Alameda County via pypums.get_acs() + as_dot_density()
- Fix projection misalignment by deriving county boundary from dissolved
  tract geometries (inline GeoJSON) instead of external topojson
- Update all output blocks (get_acs, attach_geometry, dot density) to
  match actual Census API responses (values, column order, NAME format)
- Fix choropleth legend formats: population "~s" (10M), income "$~s" ($50k)
- Pin mkdocs>=1.6,<2 to prevent incompatible mkdocs 2.x installation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chekos