If you are just starting out, the modding tutorial page is the best points to start learning. Then when you are looking for documentation of the API's, the wiki's main modding page or the main documentation index are good start points that links to all of the general documentation pages (which then link to other, increasingly specific, pages). Everything is complete and up-to-date for 1.1.
From there the primary links are:
- Documentation of the runtime API: Lists all of the properties and methods of different lua objects, and which events are available. This is one of the pages you'll want to bookmark when starting out.
- Prototypes documentation: Lists all of the prototypes that you can create, and their properties.
- Factorio data github repository: Tracks changes of the lua prototype definitions in factorio in between releases. Also useful to browse and search prototypes without opening the game files locally.
- Data.raw: Lists the names and types of all built-in prototypes.
- Data Lifecycle page: Events? Prototypes? When can use what? This page from the official API documentation explains it.