Improve documentation for advanced features of BuildStream

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/437)
In GitLab by [[Gitlab user @LaurenceUrhegyi]](https://gitlab.com/LaurenceUrhegyi) on Jun 21, 2018, 17:50

## 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 https://gitlab.com/BuildStream/buildstream/issues/278

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:

* [x] workspaces - see !519
* [x] junction elements - see !550
* [ ] split rules
* [ ] filter elements

Start by reading the existing documentation for project creation, building, and bst shell, then review issue 278 for filter-element details. Produce standalone tutorials for split rules and filter elements; completion is reflected by checking both remaining acceptance-criteria items.

Written by the indexing model from the issue text.