Rewrite the README sentence by sentence (closes #157)

The README is rewritten from scratch against the prose rules of
#157: every sentence under
about 30 words, lists as bullets, every technical term defined where it
first appears, and every statement checked against the code on next. It
leads with what RouteWatch is and how to run it, then the pages, the API
and the blocklists, then the settings, memory and upaas, and last the
design and the development workflow. The dated entry goes into TODO.md.

Model: fable-5-1

internal/server/middleware_internal_test.go

@@ -261,7 +261,7 @@ func checkAnswerCarriesMeta(
 ) {
 	t.Helper()
 
-	// The values README "API v1" gives the two fields that never change.
+	// The values README "JSON API" gives the two fields that never change.
 	const (
 		wantTimeZone   = "UTC"
 		wantAPIVersion = 1