hugolib: Don't render default site redirect for non-primary isHTML output formats

hugolib: Don't render default site redirect for non-primary isHTML output formats

When a custom output format has isHTML=true but no Path, renderDefaultSiteRedirect
would derive intermediate redirect paths from homeLink (e.g. /en/foo.html),
creating a spurious redirect at /en/ that overwrites the actual HTML content.

Only render the default site redirect for the canonical "html" format
or for formats with their own Path prefix (e.g. AMP).

Fixes #14482

Co-Authored-By: Joe Mooring <joe.mooring@veriphor.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>