Improve docs and small code cleanups (Azure, union tags, request options)

[carlos-code-max]

This PR fixes several documentation issues and applies small, low-risk code polish. It improves copy-paste correctness, consistency, and developer experience without changing runtime behavior.

Changes

• README.md

1. Fix Azure link formatting and reference URL; correct “versioning” typo.
2. Add missing imports (fmt, os) to the Azure example.
3. Fix middleware example duration calculation (use end.Sub(start)).
4. Correct union struct tag backticks in the example.

• MIGRATION.md

1. Correct union struct tag backticks in code snippets.

• CONTRIBUTING.md

1. Update stated Go version to 1.21+ to match the module requirement.

• azure/azure.go

1. Correct version string to v0.1.0 (remove extra dot).
2. Fix comment to reference option.WithAPIKey() (proper capitalization).

• option/requestoption.go

1. Use standard Authorization header capitalization.
2. Improve WithBaseURL error message (include base URL and error via %v).
3. Fix comment typo: “[]byte” instead of “[]bytes”.

Backwards compatibility

• No breaking changes. HTTP headers are case-insensitive, so capitalizing Authorization does not impact behavior.