Bring in sync with updates to datalad-extension-template

[jsheunis]

After recent maintenance to https://github.com/datalad/datalad-extension-template, the idea is to bring those changes into current extensions. This PR specifically:

• Adds the sphinx_copybutton extension to allow copying code in docs (see Add sphinx_copybutton extension to allow copying code in docs datalad-extension-template#88)
• Describes the dependabot action, cherry-picking datalad/datalad-extension-template@02d8069
• Removes an outdated appveyor comment, cherry-picking datalad/datalad-extension-template@dedc6a8
• Updates the readthedocs config filename, cherry-picking datalad/datalad-extension-template@b5c994e
• Adds a codespell config, cherry-picking datalad/datalad-extension-template@9b9e86b, and adds relevant output locations to .gitignore

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2fc957f) 94.70% compared to head (8338303) 94.70%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #252   +/-   ##
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  Coverage   94.70%   94.70%           
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  Files          24       24           
  Lines        1133     1133           
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  Hits         1073     1073           
  Misses         60       60           

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[adswa]

Thanks! I think there is no code snippet for the copy-button to act on yet, but maybe it encourages some better docs and more examples!