Improve workspace docs

Summary

As highlighted by #962 (closed) we do not cover bst workspace reset --soft in https://docs.buildstream.build/developing/workspaces.html

Details

There are many reasons to use a soft reset to trigger the configuration commands.

The doc's should cover:

• What soft reset dose
• When and how to use the soft reset

We should at least cover changing configuration options.

But maybe also highlight that in some cases adding a dependencies and running the configuration scripts will in some case cause more parts of a source to be compiled.