docs: document minification backends

[leno23]

Summary

• Document the current minification backends used by Trunk for HTML, CSS, JavaScript, SASS/SCSS, Tailwind CSS, and PNG icons.
• Note that Trunk keeps original CSS/JavaScript bytes when those minifiers cannot parse or minify a specific asset.

Fixes #810

Test plan

• git diff --check
• Searched the minification guide for the new backend entries.
• mdbook test guide was not run locally because mdbook is not installed in this environment.