docs: improve tree shaking docs

[vivekjm]

Summary

• point the bundling guide at the existing @rnx-kit/metro-serializer-esbuild documentation instead of adding a separate tree-shaking guide
• expand the serializer README with @rnx-kit/cli / rnx-bundle tree-shaking setup, serializer options, production-only behavior, and analyzer guidance

Fixes #1280

Test plan

• git diff --check
• yarn build in docsite generated static files successfully and reported existing /rnx-kit/docs/community HTML minifier warnings; the process did not return after success and was terminated manually

[tido64]

Thanks, I've left some minor comments

[tido64]

Thanks again ❤️

[tido64]

Looks like build failed because of a missing change log entry. Can you run yarn change --empty locally and push the commit?

[vivekjm]

Done, thanks. I ran yarn change --empty, pushed the empty changeset commit, and verified it locally with yarn change:check.