feat: add msgspec to python serialization guide

[v1gnesh]

Summary

Adds documentation for msgspec support in the Python SDK serialization guide.

Accompanying Python SDK PR: restatedev/sdk-python#154
Note: After seeing your follow-up fixes on the above PR, I deleted my forks, and accidentally deleted my fork of docs-restate also, closing #98.
So this is a recreation of the docs PR.

Changes

• snippets/python/src/develop/serialization.py - Added msgspec code examples (Package, CompletedPackage, ship handler)
• docs/develop/python/serialization.mdx - Added msgspec section after Pydantic (follows same structure: restate-sdk[serde] installation, usage, OpenAPI integration)

[gvdongen]

Hi @v1gnesh,

Thank you for the contribution! Looks good. I think you might still need to do uv add msgspec in the python snippets.
Once you've added that, we can merge it. Thank you!

[v1gnesh]

@gvdongen Oh?
It was added to pyproject.toml via the serde extra here - https://github.com/restatedev/sdk-python/pull/154/files; and then in this doc PR, restate-sdk[serde] is mentioned.