Write docs on load/save

[coretl]

Now that we have switched from intelligent save to intelligent load in #16, we need to write the docs for it.

I suggest:

• how-to/store-and-retrieve.md: A how-to guide on how to save and load a Device, using PandA as an example. Extending and correcting the code sample in Add save/load #16 (comment) and adding some words.

Requires #700

Acceptance Criteria

• The document is written

[coretl]

@olliesilvester were you interested in writing this?