docs: Expand v3 upgrade guide to cover all breaking changes (#698)

## Summary

- Expanded the v3 upgrade guide to document all 6 breaking changes from
the changelog: Python version support, fully typed clients (Pydantic
models for responses and parameters), pluggable HTTP client
architecture, and tiered timeout system.
- Converted both upgrading guides (`upgrading_to_v2`, `upgrading_to_v3`)
from `.md` to `.mdx` and replaced plain markdown API references with
`ApiLink` components for consistent linking to the API reference docs.
- Fixed the guide link to use the correct `custom-http-client-httpx` id.

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>