chore(deps): remove dead Elasticsearch search-index builder and unused deps (#281)

The `ext-antora/generate-index.js` extension built an Elasticsearch search
index, but it only ran when UPDATE_SEARCH_INDEX=true and the ELASTICSEARCH_*
env vars were set. The docs site aggregator no longer sets these and no longer
ships this extension (search moved to pagefind), so the code path is dead.

Removing it lets us drop its sole-consumer dependencies, shrinking the
transitive tree and the Dependabot attack surface:

- drop @elastic/elasticsearch, cheerio, html-entities, lodash (only used by
  the indexer) and the unused asciidoctor direct dep (asciidoctor-kroki stays)
- remove the generate-index.js extension and its site.yml reference
- remove the never-referenced ext-asciidoc/ext-antora/generate-index.js duplicate
- delete the orphan es-docker-compose.yml Elasticsearch dev-stack file

js-yaml and asciidoctor-kroki are kept (still used). Verified the antora build
still produces public/ output.

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DeepDiver1975