Fix catalog PDF links; add bring-online guide; feature zappier billing

PDFs rewrite in-document .md/.html links to sibling PDFs. Catalog
start page is GETTING-STARTED (install, first-up, monitor). Billing
and user platform is zappier-edge (original ~/zappier tree).

scripts/pdf-links.lua

@@ -0,0 +1,19 @@
+-- Rewrite in-document links so catalog PDFs point at other PDFs, not .md/.html.
+function Link(el)
+  local t = el.target or ""
+  if t:match("^https?://") and not t:match("zapier%.georgelambert%.org") then
+    return el
+  end
+  if t:match("^mailto:") or t:match("^#") then
+    return el
+  end
+  local path, frag = t:match("^(.-)(#.*)$")
+  if not path then
+    path, frag = t, ""
+  end
+  if path:match("%.md$") or path:match("%.html$") then
+    path = path:gsub("%.md$", ".pdf"):gsub("%.html$", ".pdf")
+    el.target = path .. frag
+  end
+  return el
+end