docs: add adoption case study template

[snoopuppy582]

Fixes #279

Summary

• Added an adoption case-study gallery page and reusable case-study template.
• Added an Adoption Story issue template for public or anonymized adopter submissions.
• Added smoke tests that validate the template, issue template, and future committed case studies.
• Linked the new docs from README, SUMMARY, mkdocs navigation, and CHANGELOG.

Tests

• uv run pytest tests/test_adoption_case_studies.py -q
• uv run ruff check tests/test_adoption_case_studies.py
• uv run pytest tests/test_docs_site.py -q
• uv run mkdocs build --strict
• git diff --check

Notes

• Kept this scoped to the adoption case-study docs, intake template, and validation coverage.