Control example order via explicit list, not via sorting titles

[drammock]

PR Description

This PR removes numbering from the titles of the examples, and switches the example ordering heuristic from relying on the (no-longer-numbered) titles, to instead relying on a JSON file containing example filenames in a fixed order. Motivation is that:

• there were two examples numbered 13
• the second of the "13"s seemed out of place at the end, but inserting it in the middle would have required renumbering all titles that came after its new place.

Safeguards:

• If a new example is created and not added to the JSON file, it will automatically sort last (not be omitted from the build).
• If an example's filename changes without updating the JSON file, it will sort last (not be omitted from the build).
• Recovering from either of the above situations is a one-line change to the JSON file (i.e. adding the new filename / updating the old filename to its new name).

closes #1320

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[drammock]

quick self-review to call out / fix a couple things

[drammock]

this is the one that was previously a duplicate "13" and appearing last. I thought it made more sense to group it with other "converting XXX to BIDS" examples.

Aside: if anyone has strong feelings about order / wants to take this opportunity to shuffle the order a bit more, feel free to push a commit changing this file.

[drammock]

NB: the JSON file won't be included in the built wheel, but that shouldn't matter as it's only needed when building our docs, so we can assume a clone of the repo in that case.

[sappelhoff]

Great, thanks a lot Dan. I like the changes, +1 to merge once CIs are green.

[drammock]

just a note that this needs sphinx-gallery/sphinx-gallery#1391 before the CIs will pass.