Keys: one table file for the device's help panel and the website's key tables (#72)

Every screen's keys are constant tables in lib/core/src/app_keys.h (52 of
them, each under an `// id: Title` comment). An App's help() picks the table
of the state it is in. site/tools/gen_dev_docs.py reads the same file and
writes site/data/keys.toml; the `keys` shortcode shows a screen's tables on
its guide page, and /guide/keys/ shows all of them.

The Site job fails when the data file is out of date or a page asks for a
table that doesn't exist, and now also runs when app_keys.h changes. A key
added to an App shows up on the website without anyone editing a page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/apps/gnss_app.cpp

@@ -1,3 +1,4 @@
+#include "app_keys.h"
 #include "gnss_app.h"
 
 #include <Arduino.h>
@@ -83,8 +84,7 @@ void GnssApp::draw(Canvas& c) {
 }
 
 void GnssApp::help(std::vector<KeyHelp>& out) const {
-    out.push_back({"Tab", sky_ ? "the position" : "the sky"});
-    out.push_back({"r", gnss_.tracking() ? "stop the Track" : "record a Track"});
+    keys::add(out, keys::kGnss);
 }
 
 void GnssApp::drawPosition(Canvas& c) {