Allow using _index.md on no _index.gmi.md

[tdemin]

This makes gmnhg use _index.md as directory / top-level index page content source in case _index.gmi.md is not present.

As the renderer has matured a lot (namely lists of links are now handled a lot better), this should allow writing almost zero content specific to gmnhg.

The docs are updated accordingly.

[tdemin]

Particularly wanted for gemini://tdem.in.