DOC: Better document "the big picture"

[pllim]

The documentation seems to have very little guidance on the "big picture" of the package. For instance:

• It does not say things like "you should use the command line tools for problem X. But the library interface is better suited for problems of class Y."
• There appears to be no real big picture on "how the code is laid out for users". How can it be easier for a new user to get a high-level picture of how to dive in and do something with the code? For example, see https://docs.astropy.org/en/stable/coordinates/index.html#overview-of-astropy-coordinates-concepts

Example user story: I am new to gravitational waves and I want to use PyCBC for my science. I have specific problem X to solve. How do I start? Which function should I use and why?