[FEA] Add docs/legend about the symbols used in B&B

[nguidotti]

Add a legend before the logs to indicate the symbols used in branch-and-bound, so the user can more easily understand the logs. Alternatively, add a note in the docs

[chris-maes]

I would suggest we add this to the documentation.

I'm moving this to 26.02, since I don't think this is critical for 25.12 release.

[anandhkb]

@chris-maes @bbozkaya What's the scope and priority here for 26.04?