Add navigation tabs to docs

[becky-gilbert]

Partially fixes lookit/lookit-docs#435

This PR adds the custom 'tabs' at the top of the CHS jsPsych navigation menu, which link to our other docs sites. These tabs match the others that we're using in the lookit-api and EFP docs.

The customization files are in _custom_theme. I've also updated the mkdocs.yml file to use this customization and to remove the unnecessary sections in the navigation menu.

Screenshots

With these changes:

Current version, for reference:

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Latest commit: 8df10d5

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[okaycj]

Look good.