Overhaul web extension docs

[JammingBen]

I feel like the web extension docs would benefit from a bit of an overhaul:

• check existing documentation for wrong information
  • https://docs.opencloud.eu/docs/dev/web/extension-system/extension-types/left-sidebar-menu-item-extensions/ code example is wrong, see https://github.com/opencloud-eu/web/blob/main/packages/web-app-office-settings/src/index.ts for an accurate one
  • ...?
• add new & missing extension points
• check if any extension types are missing
• add docs about how to build and publish extensions in the app store
• add docs about how to setup a dev environment for building extensions (using Vite and normal build mode, module federation, etc)