Further organisation of the repository

[julianstirling]

This PR hopefully ends the process of reorganising the repository.

As a summary because reorganisations can be hard to follow in the interface.. This PR:

• Moves the legacy mechanical components into the mechanical directory
• Removes the empty electronics directory
• Consolidates all of the installed modules (nimble_orchestration and nimble_builder) into a single module called nimble_build_system. This is split into three sub-modules
• Adds and updates some READMEs

Closes docs#30

[jmwright]

@julianstirling I made a few comments. Otherwise it looks ready to merge.