docs: modernise the manual stylesheet, add dark mode [skip ci]

Layer a modern look over the existing manual.css: system font stack,
larger base size and line-height, a flex header bar, lighter headings
without the blue bars, softer callouts, tables and code blocks, and a
tidier quickview sidebar, breadcrumb and footer. Content links stay
underlined (WCAG 1.4.1).

Dark mode follows the reader's OS setting via prefers-color-scheme and
is limited to screen media, so printing is unaffected.

CSS only: no markup, XML, translation or build changes.

Discussed on docs@httpd.apache.org (+1 from Rich Bowen).
Preview: https://httpd-docs-preview.jason-9eb.workers.dev/docs/trunk/bind.html