The PDF Manual has an Index (at page 1865), but the Manual itself has 2000 pages, so it is suggested to use it as a reference.
Or you can use the online Manual (Webhelp), which has a very clear Table Of Contents on the left, and a very powerful search engine:
To start learning Dorico I suggest reading the PDFs and studying the videos that you find in the Dorico Resources site:
I made a topic with some resources for users that come from Finale: (click me)