Document all NPS mapping resources investigated this session

A reference doc (docs/nps-resources.adoc) covering everything looked at
while pulling icons/controls/theme colors from NPS's mapping ecosystem:
what's actually in use (symbol-library, home-button, the Park Tiles-inspired
offline palette), what was considered and deliberately skipped (svg-icons,
markdown-handlebars, the unmerged overview/carto/wms branches) and why, the
Park Tiles cartography research (Harpers Ferry Center lineage, the four
style variants, why the actual tile service is off-limits via Mapbox
Atlas licensing, the public style JSON and Mapbox Studio style IDs found),
and the still-open upstream PR revising artwork for three icons we use.

Worth having as a knowledge base rather than re-deriving any of this from
scratch next time — NPS's tooling has repeatedly turned out to be a better
source than building something from nothing.

README.adoc

@@ -34,3 +34,4 @@ Then open http://127.0.0.1:4567 in a browser.
 * link:docs/architecture.adoc[Architecture]
 * link:docs/development.adoc[Development guide]
 * link:docs/third-party-notices.adoc[Third-party notices]
+* link:docs/nps-resources.adoc[National Park Service mapping resources]