Expose defacing code in API

[nordme]

Describe the problem

It would be great to have public-facing function for defacing MRI's. This code is implemented privately in MNE-BIDS currently.

Describe your solution

We could potentially convert the current function mne_bids.write._deface into a public function.

Describe possible alternatives

I'm open to suggestions for alternatives.

Additional context

Creating a public function for defacing would allow us to write a defacing how-to example for the MNE-Python documentation that includes the defacing functionality implemented here in MNE-BIDS as mentioned in #13695.

[larsoner]

Looks like a +1 from @hoechenberger and I'll add one as well. @nordme would you be up for making a PR for this?

[nordme]

Looks like a +1 from @hoechenberger and I'll add one as well. @nordme would you be up for making a PR for this?

Yep, I can make a PR for this. :)