docs: add migration guide for upgrading from v1 to v2 (#76)

* docs: add migration guide for upgrading from v1 to v2

This PR adds UPGRADING.md, a comprehensive migration guide for users
upgrading from the v1 Replicate Python SDK to v2.

The guide covers:
- Client initialization changes (Replicate vs Client, bearer_token vs api_token)
- Prediction lifecycle changes (resource methods vs instance methods)
- Async support differences (AsyncReplicate client vs async_ methods)
- Error handling improvements (granular exception types)
- Pagination enhancements (auto-pagination)
- Models and versions API changes (keyword arguments)
- File handling differences
- New features in v2 (models listing, better types, HTTP customization)
- Removed features (instance methods, positional arguments)
- Migration strategy recommendations

Addresses DP-684

* docs: remove unnecessary implementation detail about Prefer headers

* docs: clarify "resource methods" terminology in migration guide

* docs: add section links to migration checklist

* docs: clarify auto-pagination behavior in v2

* docs: add wait example for trainings to mirror v1

* docs: clarify file upload supports file handles without reading into memory

* docs: correct async support in use() interface - v1 also had it

* docs: add documentation reference for APIResponse object

* fix upgrading note about creating predictions

* remove unrelated comment about models.list()

* docs: update streaming section to show replicate.use() with streaming=True

Update migration guide to reflect that replicate.use() with streaming=True
is the preferred approach for streaming in v2. Mention that replicate.stream()
still works but is deprecated, without showing a code example.

* update client section of upgrade guide

* docs: use anthropic/claude-4.5-sonnet in streaming examples

* Update UPGRADING.md

* Update UPGRADING.md

* Update UPGRADING.md

Author: zeke