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 https://github.com/datalad/datalad-extension-template/pull/88)
• Describes the dependabot action, cherry-picking https://github.com/datalad/datalad-extension-template/pull/82/commits/02d8069b8a9c3f0d967f14e5c9a4d669c9d7b7ad
• Removes an outdated appveyor comment, cherry-picking https://github.com/datalad/datalad-extension-template/pull/81/commits/dedc6a8703d4e707f6ca2ee044ec4832b5dea182
• Updates the readthedocs config filename, cherry-picking https://github.com/datalad/datalad-extension-template/pull/80/commits/b5c994eb8d8c8203b3b69a99f5bf237c4623126c
• Adds a codespell config, cherry-picking https://github.com/datalad/datalad-extension-template/pull/58/commits/9b9e86b7babeb29134453486119d4fe2d326c3f6, and adds relevant output locations to .gitignore

[codeclimate[bot]]

Code Climate has analyzed commit 8338303b and detected 0 issues on this pull request.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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