docs: restructure docs for new hosting

This PR restructures the docs a little to support the new hosting in
CloudFlare and breaking away the home page management to marketting
while retaining a workable docs site.

Key changes -

- rename docs/docs to docs/guide as a path
- rename Docs menu to User Guide
- update the index in docs/guide to reflect the new name of User Guide
- remove the assets that were for partner and home page
- remove the partner page from the nav
- update the links to point to guide rather than docs where applicable
- update the mage file to remove the redunant CSS target
- update the links in the the mage docs for specialist references files

Author: owenrumney