Update docs template

[becky-gilbert]

Partially fixes lookit/lookit-docs#435 (but still need to make the same update on the CHS jsPsych page).

Summary

This PR updates the RTD theme and removes the need for the forked theme. Instead, I've used a custom template to add the navigation 'tabs' at the top of left-hand navigation menu. These tabs contain links to the other docs pages (CHS main docs and CHS jsPsych).

Here are the changes:

• Remove custom forked sphinx-rtd-theme and use standard sphinx-rtd-theme
• Add custom navigation 'tabs' with the _templates/layout.html file to match those on the main CHS docs site
• Update project title to "Lookit Ember Frameplayer" for consistency with main docs terms
• Update python requirements file
• Comment out the notes in JS code blocks (prevents build errors and improves syntax highlighting)

Screenshots

NEW - homepage with navigation tabs:

Current version, for reference:

NEW - JS code blocks with commented-out notes:

Current version, for reference: