Move foo.md to foo/_index.md where directory foo exists

We have further cases where we have both a `foo.md` an `foo/`. This results
in `food.md` being hidden. Move `foo.md` to `foo/_index.md` so that all
content is accessible, working on the assumption that the contents of
`foo/` are sub pages.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>