Option to launch just the index page in case of several pages

[paoloalba]

In case of serving multiple pages and using the --show flag, it would be cleaner to have the possibility to open just the index, and from there navigate through the pages.

An option "--show-only-index" for the panel serve command should be explicit enough

[philippjfr]

Definitely agree, and I might even go so far as suggesting that should be the default for --show.