ENH: add EGI ICAL/GCAL calibration section to read_impedances gallery example

[PragnyaKhandelwal]

Reference issue (if any)

Contributes to #13926 (Phase 5 — gallery examples and code cleanup).

What does this implement/fix?

Adds an EGI section to the existing read_impedances gallery example
(examples/io/read_impedances.py).

The new section demonstrates how to read per-channel GCAL (gain calibration)
and ICAL (impedance) data from an MFF file's info1.xml via
mffpy.xml_files.DataInfo, then visualize the impedances on a topographic
map using mne.viz.plot_topomap. Uses the existing test_egi.mff file from
the testing dataset — no new test data needed.

Additional information

[scott-huberty]

Nice @PragnyaKhandelwal ! I just push a couple small changes. I Moved the EGI section of the example to begin after the ANT Neuro section, and I dropped the vlim from the EGI topoplot. You are totally right that there is a convention in EGI systems to flag channels above 50 kΩ, but clearly this test file didn't get the memo, and so clipping vmax makes the topoplot less legible : )

[PragnyaKhandelwal]

Nice @PragnyaKhandelwal ! I just push a couple small changes. I Moved the EGI section of the example to begin after the ANT Neuro section, and I dropped the vlim from the EGI topoplot. You are totally right that there is a convention in EGI systems to flag channels above 50 kΩ, but clearly this test file didn't get the memo, and so clipping vmax makes the topoplot less legible : )

Thank you for the review and the edits @scott-huberty! I checked the rendered docs and the topomap looks much better without the vlim.... the spatial structure of the impedances across the cap looks great..
Really appreciate the feedback on the 50 kΩ clipping point, I'll keep that in mind for future visualizations!