docs(tts): add sidebarTitle 'Text to speech (TTS)' for the nav

Default sidebar label fell back to title 'Text-to-speech', which is fine
on the page header but readers scanning the Tools sidebar look for the
acronym 'TTS'. Add a sidebarTitle so Mintlify renders 'Text to speech
(TTS)' in the sidebar while keeping the canonical page title intact.

Sentence case matches the rest of the Tools sidebar group (e.g.
'Image generation', 'Music generation', 'Video generation').

docs/tools/tts.md

@@ -5,6 +5,7 @@ read_when:
   - Configuring a TTS provider, fallback chain, or persona
   - Using /tts commands or directives
 title: "Text-to-speech"
+sidebarTitle: "Text to speech (TTS)"
 ---
 
 OpenClaw can convert outbound replies into audio across **13 speech providers**