Update parameters user-guide with a real OPX getter/setter example

[marcosfrenkel]

Context

The parameters page of the protocols user guide currently has a placeholder note describing OPX behaviour hypothetically:

An OPX getter/setter would do more work: it would split the requested frequency into IF + LO, write the LO to the microwave source, and write the IF to the OPX channel. That conversion is exactly the kind of platform-specific logic the parameter abstraction is there to hold.

This was written as a hypothetical because we don't yet have an OPX implementation of qubit_tuneup (or any equivalent OPX-backed protocol) to point readers at.

What to do

Once a real OPX-backed protocol exists in CQEDToolbox (or another downstream toolbox), update the note in docs/user_guide/protocols/parameters.md (around lines 101–108) to either:

1. Replace the hypothetical paragraph with a concrete code snippet showing a real OPX _opx_getter / _opx_setter that does the IF + LO split, mirroring the structure of the existing DUMMY/QICK example, or
2. Move it into the "Real-world parameters" section as a second worked example alongside the existing nestedAttributeFromString snippet.

Option 1 is probably the cleaner edit — the note's job is to ground the "hardware translation" claim, and a real example does that better than a hypothetical.

Type

Documentation follow-up — no code change to labcore itself.