DOCS: Add GitHub support to copyfiles, remove online notebook downloads

[dwhswenson]

In order to move notebooks into the right locations, a while ago I added a copyfiles script to our docs directory. This is specifically used to bring our docs from examples into the docs, where Sphinx can find them.

Today, I discovered that it was using GitLab to download (very outdated) copies of our tutorial notebooks. So I updated the script to allow us to use GitHub instead.

Then I realized that I we're not even using those downloaded notebooks in our docs. 🤷‍♂️ It definitely makes more sense to just link to the repositories, especially given the option of launching the tutorial with Binder.

In any case, I left the addition of support for GitHub, and removed the downloads of the tutorial notebooks from online (this also would have been an annoyance to anyone trying to build our docs without internet connectivity). I left previous usage commented as an example for any future users.

This should be immediately ready for review.

[sroet]

LGTM

[codecov]

Codecov Report

Merging #1087 (60eb8eb) into master (0ca38e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1087   +/-   ##
=======================================
  Coverage   81.56%   81.56%           
=======================================
  Files         140      140           
  Lines       15452    15452           
=======================================
  Hits        12604    12604           
  Misses       2848     2848           

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