Document Lucene benchmark backend

[nvzm123]

Note

Draft 5 of 5 in the Lucene backend split. It documents the final stacked
behavior after the implementation and packaging layers.

Summary

• add a dedicated Lucene backend guide;
• nest Lucene beneath the Fern Backends navigation;
• list all three supported Lucene algorithms in the running guide; and
• document runtime prerequisites, the source-builder workflow, timing boundaries,
  backend selection, and opt-in live integration tests.

Validation

• fern/docs.yml YAML parsing and navigation-target checks: passed;
• fern/build_docs.sh shell syntax: passed.

fern/build_docs.sh check was not runnable because Node.js 22 or newer is not
installed on this host.