initial draft of new TD-fNIRS KF2 example

[JohnGriffiths]

Following from this PR discussion, this example demonstrates I/O of TD-FNIRS data, and also serves as an introduction to KF2 data.

Code and data based on example code from @julien-dubois-k

Remaining to-do:

• Modify installation target to the above-linked PR
• Add data download functionality
• Modify some of the hdf access bits with references to getting the info from the mne snirf loader

[JohnGriffiths]

PR adding KF2 tutorial example, as discussed.

I have verified that this builds an correct-looking example webpage with Make html

@larsoner - do need some help here with getting the CI to pass

Some things to note re: the contents of this example:

1. It currently uses gdown to pull the example file. I suggest we keep this as-in, and consider updating to something with mne-misc-data after this one is merged

2. Likewise, I suggest we edit the mne dependency back to the mne-tools/mne-python master in a later PR, once the PR in the above-linked thread has been resolved.

3. Some of the contents regarding pulling stuff from the hdf5 archive could be modified in future versions to show pulling this info from the hdf archive directly or from the mne reader. However I don't believe this functionality is in the MNE .snirf reader yet, so we would need to complete a new PR with that code first.