Unified sphinx theme project

[Stuart Mumford]

Hello all,

I've been working on my cycle 5 funding request (https://github.com/astropy/astropy-project/blob/main/finance/proposal-calls/cycle5/aperio-documentation.md) to roll out a sphinx theme with consistent navigation over multiple astropy projects, and various elements of it are now up for review.

Please see this issue which links all the work and what is complete and in need of review: https://github.com/astropy/astropy-project/issues/522

Thanks,

Stuart

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