[ecosystem] Reorganize Add/Update pages

[chalin]

Following work on:

• [ecosystem] Document registry curation policy and responsibilities #5937

I'm feeling that we should pull the "How to add" and "Updating" pages from under the registry section, to place them directly under /ecosystem. WDYT?

[tiffany76]

Another option would be to make the Registry index page a landing page with three child pages: the registry itself, adding, updating.

Either way, I agree that those two pages get lost in the current architecture. If you click on Registry, you lose the menu and might forget to go back to look.