docs: refresh README with the Strands wordmark

[pgrayy]

Description

Align the README with the deployed Strands branding. The old shared logo URL still serves the ribbon, so this uses the new wordmark with light and dark variants.

Related Issues

None.

Documentation PR

Included in this PR.

Type of Change

Documentation update

Testing

Rendered the header through GitHub’s Markdown renderer and checked both color variants in a browser at 390px and 1100px widths. The SVGs load, the logo scales without horizontal overflow, and git diff --check passes.

• I ran the relevant test suites for the bindings I touched (cargo test --workspace --all-targets, pytest tests/python, npm test)
• If I touched Rust, I ran cargo fmt and cargo clippy

Runtime suites were not run for this README-only change.

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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