Add GitBuilding Syntax for for top-down modular hardware

[julianstirling]

This issue describes a task that forms part of the Nimble smart doc project funded by the NLNet foundation. This work will enable work within this project but will happen in the GitBuilding repository

[julianstirling]

This has been identified as a shortcoming of GitBuilding for over 2 years
https://gitlab.com/gitbuilding/gitbuilding/-/issues/227

The core implementation of the top down syntax was in this Merge request
https://gitlab.com/gitbuilding/gitbuilding/-/merge_requests/187

For the 3D printing page it was necessary that the contents of the page itself had to be updated depending on which parts were printed. This is a generic problem for any type of documentation that has a number of pieces being made by a manufacturing process. It could be CNC machining or 3D printing, or a something a simple as a cutting list for timber. As discussed in the comments of the issue thread linked above. It was decided to add the data of the components that need to be manufactured to the YAML header of the page that is using them. A template for the manufacturing process can then be written to create the page that the reader will see. This was implemented in the following merge request:
https://gitlab.com/gitbuilding/gitbuilding/-/merge_requests/195

Also merge requests https://gitlab.com/gitbuilding/gitbuilding/-/merge_requests/194 and https://gitlab.com/gitbuilding/gitbuilding/-/merge_requests/196, were bug fixes related to the implementation of this syntax.

This new syntax has then been tested using the exampled documentation repository for the nimble project
Wakoma/nimble-example-documentation#1