A few improvements I'd like to request for the Extending eZ Platform guide:
1. The page itself is a guide about customizing the Platform Admin UI. This part be a subchapter of that section, dedicated to it. It would help SEO, and would allow to keep the index for other purposes
2. Write a new index page that a) introduces platform extensibility (bundles, events in general...) and b) lists the sub-guides (Admin UI, blocks, etc) with a short "what for ?" for each
3. Add Field Types there. A link to the tutorial could do, but this particular (very important) extension point deserves some clarity