feat(field-guide): an evergreen explainer at /field-guide/about + a tighter entry blurb (#85)

The hub's "learn more" link pointed at the dated launch blog, a snapshot frozen
at the drivers-first moment that drifts as the guide grows. Add a canonical
evergreen page (a "specimen tour" in the trust-model tier: guilloché hero well,
Cormorant chapter marks, the three real specimen languages, AccessDepth core
samples, the master seal) and repoint the hub at it. …