Make observers metadata / storage publicly visible

[alice-i-cecile]

Objective

Our strategy for storing observers is made up of several moving parts, which are ultimately fairly simple nested HashMaps.

These types are currently pub, but lack any meaningful way to access this data.

We have three options here:

1. Make these internals not pub at all.
2. Make the data read-only accessible.
3. Make the data mutably accessible.

Solution

I've opted for option 2, exposing read-only values. This is consistent with our existing approach to the ECS internals, allowing for easier debugging without risking wanton data corruption. If one day you would like to mutably access this data, please open an issue clearly explaining what you're trying to do.

This was a pretty mechanical change, exposing fields via getters. I've also opted to do my best to clarify some field names and documentation: please double-check those for correctness. It was hard to be fully confident, as the field names and documentation was not very clear ;)

Testing

I spent some time going through the code paths, making sure that users can trace all the way from World to the leaf nodes. Reviewers, please ensure the same!

Notes for reviewers

This is part of a broader observer overhaul: I fully expect us to change up these internals and break these shiny new APIs. Probably even within the same cycle!

But clean up your work area first: this sort of read-only getter and improved docs will be important to replace as we work.

[Vrixyz]

you changed trigger to event on a precedent line, but here trigger is back, it's a bit confusing ; maybe doc linking to actual type may help, so documentation for what those terms refer to would be easily reached and consulted 🤔

[Vrixyz]

a minor comment on clarification needed regarding trigger vs events, but it's difficult to make better actionable feedbacks, I think this PR is useful and in a good enough state so I'm approving.