Add support for composing MyST configurations

[agoose77]

Proposal

In a meeting with @rowanc1, it was mentioned that journals might wish to define base configurations that e.g. set attribution information. Rowan also pointed out the value to satellite-like organisations such as 2i2c's community work to define a "base" set of acronyms, glossaries, or other features and have derived MyST projects inherit them.

I think this feature has pertinence to Jupyter Book 2's configuration, which may be as simple as defining a base JB set of defaults.

We should consider a new field in the configuration, e.g. overlays or composes that defines a set of URIs that resolve to configuration files.

Whether JB 2 still chooses its own jupyter-book.yaml that abstracts this mechanism remains to be seen.

[choldgraf]

Oooh yes this is great - this would I think solve the internationalization issues. I also think it leans into the MyST idea of "one source of truth" applied to the config layer.

For one example, I could see 2i2c using this to configure standardized navigation links across all of its websites by defining them in one repository, and re-using them in others.

I like the idea that a Jupyter Book specific config is effectively using this feature as a starting point, and letting users specify their own config (or overrides) in addition.