V1 Docs: Add instructions for creating your own starter theme

[Erin-Cecele]

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Idea being proposed

We've recently versioned the default starter theme, which opens up the possibility of creating your own starter theme, like a template. You would have the option to use the default generated by quire new but also the ability to create your own. This needs to be documented.

What contribution category does your proposal fall under?

Propose/write new section

Supporting Information

No response