[docs]: Add expand documentation

[BenJamesBen]

Where can we see the problem? 🤔

No response

Are you a FreeSewing patron? 😃

Yes, I am a tier-2 patron ❤️

Suggested content

There is a newly-added expand setting that needs to be documented. This setting is also exposed in Part shorthand and needs to be added to the docs there. Finally, we should write a Howto to provide practical instructions to pattern designers and coders how to use the setting in their source code to conditionally hide parts or present alternate parts.

https://freesewing.dev/reference/api/part/draft
https://freesewing.dev/reference/settings
https://freesewing.dev/howtos/code

Additional context

No response