Legacy cleanup on the AI reference pages #578 touched

[hey-august]

All pre-existing and left alone during #578 under the leave-untouched-text-alone rule, but each now sits
next to rewritten text.

• languages/index.mdx:2-4: legacy frontmatter (id: 6278079e-…, unlisted: false, hide_title: false).
• languages/index.mdx:58 and :197-201: the bare-voice-id warning appears twice on one page. Keep the
  <Warning>, shorten the first to a cross-reference.
• languages/index.mdx:76, :90, :172, :176: default="None" where the source defines no default.
  Document ai.multilingual and qualify voice strings with their engine #578 corrects the ParamFields it rewrites; the rest is a page-wide pass.
• ai/prompt/index.mdx:161-171: the filler resolution order is stated at :164, restated at :170, and
  linked at :170 to the canonical treatment that states it a third time. Cut one same-page restatement.
• multilingual/index.mdx:120-134: fillers and function_fillers share two verbatim-identical
  sentences. The second could compress to "Same shape as fillers", the way exit_fillers does.
• server-sdks/pages/guides/build-ai-agents/voice-language.mdx: body headings start at ###, headings
  are Title Case, the frontmatter title uses &, and the python fences at :68/:71 use ## for
  comments where # is idiomatic.

Done when: each is fixed or deliberately left, page by page.