Support an explicit index.md group overview page

The section index can now be authored either as <Group>/<Group>.md (mapped to index.md) or as <Group>/index.md directly (published as-is). If a folder has both, the explicit index.md wins and the <Group>.md stays a normal page. Fixture: SomethingElse now uses index.md to cover the passthrough path (PSModule still covers the mapped path).

Author: MariusStorhaug