Stack the home-page animations on phones

[basnijholt]

Description

On phones, the three animations on the docs home page were shown side by side at 43%, 36%, and 19% of the screen width, which made them too small to read. Below 45em (Zensical's phone breakpoint), they are now stacked at full width. The desktop layout is unchanged.

Checked in headless Chromium: at 390 px wide each GIF is 343 px wide and they sit below each other; at 1400 px they stay in one row.

This only affects the docs; the README on GitHub cannot use custom CSS.

Checklist

• Fixed style issues using pre-commit run on the changed file
• pytest passed (CSS-only change; zensical build --strict reports no issues)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update