MRG: Add fNIRS to html output

[rob-luke]

Reference issue

None. But I really love these html outputs in notebooks, so cool.

What does this implement/fix?

Add fNIRS to the html output for raw.

Additional information

I follow #9172, but am unsure how to test this change. Can we merge without a test?

Alternative solution

If you don't want to add fNIRS channels as they are less common we could have an optional row using:

    {{if n_fnirs > 0}}
        <th>fNIRS channels</th>
        <td>{{n_fnirs}}</td>
    {{endif}}

I tested this locally and it works too, I could do the same for EOG and ECG too if you want.

[rob-luke]

Render of change can be viewed here: https://28136-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/preprocessing/70_fnirs_processing.html#preprocessing-functional-near-infrared-spectroscopy-fnirs-data

And here is a screen grab...

[rob-luke]

@larsoner @drammock @agramfort are you ok with this or would you prefer my alternative option above?

[agramfort]

thx @rob-luke