Summary

• Resolve generated docs breadcrumb hrefs against registered docs routes.
• Fall back parent breadcrumb links to /overview/ when the synthesized parent path is not a route.
• Add a regression test for the Enterprise breadcrumb case from the breadcrumb base links aren't working on [reflex.dev](http://reflex.dev/docs) #6437.

Root Cause

The docs breadcrumb builder created parent links by concatenating path segments. For Enterprise docs, that produced /docs/enterprise/, but the real route is /docs/enterprise/overview/.

Validation

• uv run pytest tests/test_breadcrumbs.py tests/test_routes.py -q
• uv run ruff check reflex_docs/templates/docpage/docpage.py tests/test_breadcrumbs.py
• Verified locally in Browser that clicking the Enterprise breadcrumb lands on /docs/enterprise/overview/.

Fixes #6437