Document Elasticsearch exporters

## Description
Elasticsearch exporters (es and semantic) are now supported in v3, but they are not documented anywhere. This makes it difficult for contributors to understand what they are, why they exist, and how to use them.

Currently, the exporters:
- Can be specified as options to docs-builder and docs-assembler.
- Handle indexing documentation into Elasticsearch (with or without semantic fields).
- Automatically bootstrap inference endpoints, settings, and mappings (no manual setup required).

## Requested documentation

- Overview:
- What exporters are, why they exist, and when to use them.
- Difference between es and semantic.
- Usage examples
- Using exporters with docs-builder to index docs from a single repo.
- Using exporters with docs-assembler to index all published docs.
- Example environment variable setup (DOCUMENTATION_ELASTIC_URL, DOCUMENTATION_ELASTIC_APIKEY).
- Details + behavior
- What happens under the hood (inference endpoint, settings, mappings bootstrapped automatically).
- Limitations or caveats (if any).
- Troubleshooting
- Common errors and how to resolve them (e.g., API key issues, missing environment variables).

## Acceptance criteria
- Documentation page(s) exist that explain Elasticsearch exporters in plain language.
- Writers and contributors can successfully use exporters with both docs-builder and docs-assembler without developer intervention.
- Examples provided are copy-pastable and verified to work.

## Impact
- Reduces confusion among contributors.
- Makes semantic search testing/adoption across /docs straightforward.
- Removes dependency on engineering for setup.