West documentation is largely missing

[mbolivar]

The documentation for west has several inaccurate statements when it comes to the multi-repo commands, and is missing entirely for several features:

• configuration
• extension commands in general
• the sign extension command

Additionally, API documentation is missing.

Since west is part of the getting started guide now, we need to fix this so users can learn how to use it without having to make guesses based on online help and the source code.