Index code blocks in the docs search

[cdebled]

Code blocks are excluded from the search index, so the reference section is effectively unsearchable.

• Nextra defaults search to { codeblocks: false }, which puts data-pagefind-ignore="all" on every <pre>. Pagefind skips their contents.
• The reference pages are mostly code blocks. /reference/cli/serve contributes 180 indexed characters, and no flag name (--host, --port, --protocol) is in the index at all.
• Tradeoff: the option is global, so every snippet on the site gets indexed. Bigger index, more noise on tokens like await.