docs: document beam-large-records

[ColumbusLabs]

Summary

• document when beam-large-records is useful for schemas with many fields
• show the plugin, integration import, and largeRecord annotation
• list the direct dependencies required by the plugin and link authoritative package documentation

Closes #825.

Validation

• verified the package metadata and example against the official beam-large-records source
• verified the quoted benchmark values against the upstream timing data and methodology
• verified all three added links resolve
• checked Markdown fence balance
• git diff --check

The repository's nix build .#docs -L check was not available locally because Nix is not installed.

[LaurentRDC]

Thank you!