docs: rewrite prompting guide and add downloadable reference (#1049)

* docs: rewrite prompting guide and add downloadable reference

Rewrites the prompting guide with voice-specific best practices
(six-section structure, guardrails, few-shot examples, anti-patterns).
Adds a downloadable .md reference file for use with Claude Code or
other AI coding assistants. Fixes broken links, adds internal
cross-links to related docs pages, and cleans up formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: use Fern Download component for prompt reference file

Replace Card href with Fern's built-in Download component which
handles file downloads correctly via relative src path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add advanced voice agent techniques section

Adds "Making your agent sound human" section covering disfluency
design, rapport patterns, banter vs off-topic handling, energy
matching, turn budgeting, emotional expression frequency, incremental
tool calls, read-back strategy, and call ending rules. Informed by
patterns from the Vapi qualification demo agent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: update pronunciation advice — prompt hints can help

Pronunciation hints in the prompt can help depending on the voice
provider. Updated both files to recommend using prompt-level hints
alongside TTS-layer pronunciation dictionaries, rather than saying
prompt hints don't work.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: clarify pronunciation dictionaries are ElevenLabs-only

Makes it clear that TTS-layer pronunciation dictionaries are
currently ElevenLabs-only, so prompt-level hints are the primary
pronunciation tool for other voice providers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: remove incorrect ElevenLabs-only claim for pronunciation dicts

The pronunciation dictionaries page only documents ElevenLabs but
other providers may also support them. Reverted to provider-neutral
language.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: betzlermeow