Restructure docs nav, add Models page, remove Sentences

[egenthon-cmd]

• Adds an Introduction Models page that compares Solaria-3 vs Solaria-1 (default): when to use each, language/mode limits, SDK + cURL examples, and how language_config works without code switching on Solaria-3.
• Moves Playground and Benchmarking into a new How to use Gladia nav group, and adds redirects from the old URLs.
• Removes the Sentences feature page and leftover links (features index, recommended parameters, translation).

[mintlify]

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Project	Status	Preview	Updated (UTC)
gladia	🟢 Ready	View Preview	Aug 17, 2026, 7:15 PM

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