Restructure developer guide and add architecture overview

[stefanv]

Hi folks 👋!

I'm a developer from the Scientific Python ecosystem, who is new to mystmd, but who has some experience in building documentation tools. Such as the original SciPy Proceedings machinery, that y'all have now replaced with something so much better :)

I am a maintainer of the Scientific Python Hugo Theme, used for projects like NumPy, SciPy, etc. We are investigating moving from Hugo to mystmd, and I'm learning the mystmd stack to determine what that pathway looks like. I.e., I am particularly interested in myst-theme and landing pages.

As I'm learning, I thought it might be useful to help update the developer onboarding material, to soften the landing for those who come after me.

I must apologize in advance: it is entirely possible that I may still have some fairly fundamental misunderstandings, but I hope the proposed structure / concepts / edits are useful, and that we can iterate to find some useful additions. I am not attached to any of the material I submit here, and happy to pivot at your direction.

I've marked all areas that I know need feedback with an error directive, so you can easily locate them. Thank you in advance for your feedback!

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