Add developer documentation for managers in NEST

[jessica-mitchell]

This PR creates pages dedicated to explaining how managers work in NEST.
The goal is to provide newer developers with an overview.

Currently a general overview is given in developer_space/managers/index.html
And each manager has its own page with a brief description and generated docs from doxygen output (extracting the C++ comments) using the extension breathe. Additional details are provided for MUSICManager and SPManager, after discussion with @alpinangu on what would be useful to include. If the review of these pages is positive than we can include such details in each manager page.

The analysis of code and text descriptions was aided by AI.

[alpinangu]

Hi Jessica, I think the changes you made are really helpful.

I really like the Relationships with Other Managers section. I think it’s super useful for someone who doesn’t have a clear big-picture understanding. This section provides that well from the manager’s point of view. I also like that there are links to the listed managers.

I also really like the detailed operation sequence. The explanations are clear and well-ordered, and I appreciate how it includes the code too—so you can look at the function without needing to switch tabs. Maybe it could also mention which parts of the source code call these functions, if it’s not too hard to trace.

I also really liked how MUSICManager has a section for MPI, since both it and MUSIC rely heavily on it.

The Overview section and the description at the top also provide a nice explanation. It’s helpful for people who just want a quick reminder instead of going through the detailed breakdowns.

It might be helpful to include a super brief description of what the class members are for. Personally, when I start working on a class, I like to understand the members before diving into the functions.

Lastly, I found that some of the Doxygen descriptions could be more detailed.