Add a more complex example

## Background
The current example project in the documentation is a good example of how to start a new project and it demos important things such as creating a new project, using various commands, and using various element types. However the project doesn't bridge the gap between a basic hobby project and using Buildstream in a realistic manner.

## Problem Description
Specific areas I believe that I believe are particularly missed out on are:
- Junctions are poorly explained.
- The interaction between Junctions and mirrors is not well demoed.
- Overrides are barely touched upon.
- Mirrors are mentioned, but the example is far off actual usage.

## Definition of Done
Therefore I think adding a complex example into the Buildstream docs which goes through:
- Junctioning from Freedesktop-sdk
- Building a complex item from a mirrored git-repo
- This should include good examples of Overrides
- This should include good examples of git CI/CD usage
- This should include good examples of Debugging

Start by locating the current example project in the BuildStream documentation and reviewing how it presents project creation, commands, and element types. Define the scope of a more realistic example covering Freedesktop-sdk junctions, mirrored Git repositories, overrides, Git CI/CD, and debugging. Done means the documentation includes and explains that complete complex example.

Written by the indexing model from the issue text.