Modify styling of main docs content

[jameshadfield]

This tweaks the CSS used by the main section of the docs pages (not the sidebar). Styling is largely subjective, but this improves the readability of the text (heavier weight, more spacing between lines, more contrast between text and background). It also improves the aesthetics of the inline- code sections and the buttons at the bottom of each page.

The styles here are inspired by the current nextstrain.org/docs and auspice's docs (which I really like).

How to run

• checkout this branch
• switch to the conda environment docs.nextstrain.org
• run pip install -e . to replace the published theme with this one
• build the nextstrain.org docs as normal, they'll use this updated theme :)
• to return to the old version either switch to the master branch or run python3 -m pip install 'nextstrain-sphinx-theme>=2020.2'

(There may be a better way -- please let me know if so)

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This represents my first forays into sphinx and the theming it uses. I hope to further modify the underlying HTML used for rendering.

[jameshadfield]

Before & after screenshots:

[eharkins]

Would this apply to subprojects of docs.nextstrain.org if we merged it and it was being used by that project or would we need to update the theme for subprojects separately?

[tsibley]

@eharkins For the RTD projects to use these changes, the following must happen:

1. Merge this PR.
2. Release new version of this theme package to PyPI.
3. Wait for RTD projects to rebuild naturally or manually trigger new RTD builds.

If any of our RTD projects declared exact versions of this theme instead of minimum versions, then those would have to get bumped. But I don't think any of our projects currently do this.

[tsibley]

Worth nothing that both 2 and 3 could be automated via GitHub Actions or Travis CI if we desired.

[jameshadfield]

Thanks both! Going to merge this (and #2) for the upcoming merge of the docs.nextstrain.org migrate branch