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>

README.md

@@ -70,4 +70,4 @@ Edit `SITE_URL` in `.env`, then `docker compose up -d --build`. Docusaurus bakes
 
 A handful of "See Pro" / "See free plugin" cross-links inside the docs were written for the original plugin repo structure and were adjusted during setup — all internal links were checked and resolve correctly as of this build. If you reorganise the `docs/` folder structure later, re-check cross-plugin links (the free and pro plugin folders link to each other in a few places, e.g. product-attribute-colours ↔ the Pro widget that displays swatches).
 
-The marketing/"what it does" copy (as opposed to this how-to content) is intended for regular Elementor pages on dotjuice.co.uk, not this docs site — see each plugin's `documentation/marketing/` folder in its own repo.
+The marketing/"what it does" copy (as opposed to this how-to content) is intended for regular Elementor pages on dotjuice.co.uk, not this docs site — it lives in this repo under `marketing/<plugin>/`. The copies in each plugin repo's own `documentation/` folder are abandoned. Those pages pull the markdown straight from this repo's raw URLs via the free plugin's Markdown widget / `[dj_markdown]` shortcode, so a commit here updates the website copy.