docs: document CSS decimal precision difference between Turbopack and webpack (#91909)
## Description
Documents the CSS decimal precision difference between Turbopack and
webpack in the Turbopack docs page under "Known gaps with webpack".
Turbopack uses Lightning CSS which outputs 5 digits of decimal precision
for numeric CSS values, while webpack uses 10 digits. This applies to
both plain CSS and Sass/SCSS output.
Closes https://github.com/vercel/next.js/issues/91862