PHP code blocks in the mkdocs documentation have no syntax coloring

Problem/Motivation

PHP code blocks in the mkdocs documentation have no syntax coloring. Two issues cause this:

1. The Pygments PHP lexer expects code to start with <?php. Without it, everything is treated as plain text — no highlighting tokens are generated.
2. The Material theme applies its own colors via CSS custom properties (--md-code-hl-variable-color, etc.) that default most tokens to gray. The original pygments.css used bare selectors (.nv, .na, etc.) which lose the specificity battle against Material's scoped selectors.

Steps to reproduce

1. Build the mkdocs site: mkdocs build
2. Open any page with a PHP code block (e.g. Domain > Path prefix)
3. All PHP code appears in a single gray color — no distinction between keywords, variables, strings, comments, etc.

Proposed resolution

mkdocs.yml: Configure pymdownx.highlight with extend_pygments_lang to pass startinline: true to the PHP lexer, so PHP code blocks are highlighted without needing <?php. Also enable pygments_lang_class to add language classes to the wrapper div.

pygments.css: Rewrite with CSS nesting under .highlight to increase specificity and override Material theme defaults. Lighten comment color from #3d7b7b to #4f7676 for WCAG AA contrast compliance.

Remaining tasks

Review and commit.