Flatten marketing paths ahead of wiring up the websites

Drop the redundant documentation/marketing/ nesting inherited from the
plugin repos, so files sit at marketing/<plugin>/<page>.md. These paths
become raw URLs pasted into Elementor pages and ACF fields, so they are
much cheaper to change now than once sites reference them.

Fixes the four cross-plugin links for their new depth, and corrects the
README, which still pointed at the abandoned per-repo documentation/
folders instead of this repo.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

marketing/dotjuice-pagespeed/advanced.md

@@ -0,0 +1,15 @@
+# Advanced
+
+Server-side tuning for sites that want to squeeze out every remaining bit of overhead.
+
+## The problem it solves
+
+Beyond what shows up in a page's HTML, WordPress runs background processes that cost server resources regardless of whether a specific site actually benefits from them — a dashboard polling mechanism firing every fifteen seconds, a legacy remote-publishing protocol most sites never use, and WooCommerce's own scripts loading in full even on pages that have nothing to do with shopping.
+
+## How it helps
+
+Dotjuice Pagespeed lets you throttle or disable WordPress's Heartbeat API (the background polling that powers autosave and "someone else is editing this" notices), disable XML-RPC if you don't use remote publishing tools, and stop WooCommerce's cart and shop scripts from loading on pages that aren't part of your shop.
+
+## The result
+
+Less background server load, and WooCommerce's overhead confined to the pages that actually need it.