fix: redirect broken /docs/features/speech-to-text link + rewrite upload_as_text docs (#539)

Closes #430 — adds redirect from /docs/features/speech-to-text to
/docs/configuration/stt_tts in next.config.mjs.

Also rewrites the upload_as_text.mdx page from scratch:
- Replaces AI-sounding boilerplate with direct, human language
- Uses fumadocs components: Steps, Tabs, Cards, Accordions, Callouts
- Adds practical decision guide table for choosing upload method
- Moves admin-only config into collapsible Accordions
- Restructures troubleshooting as expandable FAQs
- Keeps all technical accuracy (MIME matching, priority chain, etc.)

Author: berry-13