docs: add missing PostCSS install steps to "start from scratch" guide

[anishesg]

The "Start a site from scratch" Hugo-module guide in
docsy.dev/content/en/docs/get-started/docsy-as-module/start-from-scratch.md
was missing the PostCSS installation step and postcss.config.js creation,
causing users to hit a Cannot find module autoprefixer error when running
hugo server on a freshly created site.

The TL;DR command blocks (both Unix and Windows) now include
npm install -D autoprefixer postcss-cli postcss and the creation of
postcss.config.js with the autoprefixer plugin configuration. A new
"Install PostCSS" subsection was also added to the Detailed Setup instructions
with the same commands and a cross-reference to the
installation prerequisites page.

Fixes #1718

[chalin]

Thanks @anishesg.
@deining - thoughts?

[anishesg]

Fixed the prettier CI failure — the postcss.config.js snippet had 4-space indentation instead of 2 and was missing the trailing semicolon/comma. Ran npx prettier --write to bring it in line.