docs: sync CN migration hotfixes

[Foesa-yyx]

Summary

• Sync the CN documentation hotfixes that were confirmed missing after the migration cutoff in AgoraIO/shengwang-doc-source.
• Cover RTC, RTM 2.3.0, Conversational AI and Agents SDK, Speech-to-Text 7.2.3, Cloud Recording, multi-usecase billing, and the four content-audit hotfixes.
• Add the missing custom TTS pages and screenshots, and align the affected OpenAPI sources, SDK download data, API descriptions, and nested list structure.

Audit scope

• Included: AgoraIO/shengwang-doc-source#2006, #2028-#2031, #2033-#2049, #2052-#2054, #2057-#2058, and #2061.
• #2006 is included because it merged late and its RTM 2.3.0 updates were not present in CN-NEWDOC.
• #2052, #2053, #2054, and #2057 were initially excluded, then migrated after a second comparison confirmed their exact changes were absent from CN-NEWDOC plus this PR.
• Skipped AgoraIO/shengwang-doc-source#2032: its Safari support update for getPlaybackDevices is already present in the CN-NEWDOC API page.
• Excluded source-only generated release artifacts and agent knowledge-base metadata.
• The source history was rechecked through 474985f901fffffd4ffed27ed35a060f986bdb98; no later merged PR or first-parent commit exists at the time of the final audit.

Verification

Initial batch:

• git diff --check
• Parsed both modified OpenAPI YAML files successfully
• bun run openapi:sync
• bun run types:check
• bun run docs:links
• bun run build (3,227 prerendered pages)
• OpenAPI focused tests: 9 passed
• Updated SDK assertions: 3 passed
• MDX syntax baseline comparison: 58 existing findings before and after, 0 new findings

Additional content-audit hotfixes:

• git diff --check
• bun run types:check
• bun run docs:links (exit 0; existing repository findings remain)
• All 8 affected local routes returned HTTP 200
• Visually checked the updated iOS background screenshot and the Android nested recovery/token lists

The full test suite is already red on clean CN-NEWDOC with 7 pre-existing failures. The full SDK test file still reports the existing version-selector test that expects a combobox for a single-version card; the assertions changed in this PR pass.