Replace usage of `@import` in CSS with documented recommended CSS imports

[spwoodcock]

Describe the bug

Related to #177

• Using @import within a stylesheet causes a download waterfall.
• Instead of stylesheets loading in parallel, they are loaded in sequence.
• This causes poor performance on first page load, and even flash of unstyled content.
• This is particularly apparent from the google font imports.

Alternative

• Instead of doing this, we should probably just document well which imports should be included in any app that runs hotosm/ui.
• This will allow all stylesheets to load in parallel and be cached nicely in browser too.
• Overall, at the expense of a little better documentation, we have a much nicer solution.

[linear]

TECH-1008