Add ecosystem package documentation page

[xushenbo]

PR Body

PR Checklist

• This comment contains a description of changes (with reason)
• Referenced issue is linked
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

This PR adds a small ecosystem documentation page to describe specialized packages that build on ehrapy without expanding the core API.

The immediate motivation is reviewer feedback on a self-controlled cohort drug-screening workflow based on Foundational Model-aided Automatic High-throughput Drug Screening Using Self-controlled Cohort Study. Rather than adding that setup-specific workflow directly into ehrapy.tools, the workflow now lives in the separate ehrapy-drug-screening ecosystem package with its own implementation, tests, documentation, and tutorial notebooks.

The new docs page explains that the future ecosystem package is intended to cover:

• exposure episode construction
• self-controlled cohort drug-screening analyses
• grouped screening across drug hierarchy levels
• indication mapping
• post-processing
• optional LLM-assisted review

Technical details

This PR intentionally keeps ehrapy focused on foundational EHR analysis. It does not add the full drug-screening implementation to ehrapy-main; that work is hosted separately in a standalone ecosystem package generated from the scverse package template.

Main changes:

• Adds docs/ecosystem.md
• Links the ecosystem page from docs/index.md

Validation performed locally:

• git diff --check

Docs build note:

• The ehrapy-main docs build was not rerun after this docs-only change because the local ehrapy-main environment is missing sphinx_tabs. The ecosystem package itself was validated with pytest and a Sphinx -W docs build.

Additional context

This PR is a scoped documentation update that aligns with the reviewer recommendation: keep the specialized drug-screening workflow in a dedicated ecosystem package, and reference that ecosystem expansion from ehrapy documentation.