Add new TTS language parameters to SWML schema and docs

[hey-august]

Documentation Update Pull Request

Todo

• Add note that languages[].params aren't supported by Amazon, Polly, and GCloud
• Link to stability/similarity from ElevenLabs doc

Description

Update SWML schema and docs to reflect a new ai.languages.params object, and its two properties:

• languages[].params.similarity
• languages[].params.stability

Deprecate the old controls in ai.params:

• params.eleven_labs_similarity
• params.eleven_labs_stability

Preview

Related issue: #684

Type of Change

• New documentation
• Update to existing documentation

Checklist:

• My documentation follows the style guidelines of this project
• I have performed a self-review of my documentation
• My changes generate no new warnings
• Builds successfully locally

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