Improve documentation for advanced features of BuildStream

Background

Now that we have improved the documentation for newcomers to the project, we should focus on improving the documentation for the more advanced features that BuildStream has.

Task description

We already have the basics of creating and building a project, and use of bst shell. Important topics still to cover are:

• workspaces
• junction elements
• split rules
• filter elements - also see further details at #278 (closed)

Each item should be a stand-alone tutorial.

We should also include more examples, but I think that should be covered separately to this issue.

Acceptance Criteria

There should probably be a corresponding MR to match each of the sub-headings above, ie:

• workspaces - see !519 (merged)
• junction elements - see !550 (merged)
• split rules
• filter elements