Align menu with rest of ecosystem

[MarcSkovMadsen]

Request

Align hvPlot menu with Panel, HoloViews and rest of ecosystem.

Motivation

Will make navigation across the ecosystem easier.

As Is

The hvPlot menu looks like

The HoloViews (and Panel) looks like

To be

Rename Topics to Gallery and move it before reference gallery.

If possible also use nice gallery template used by HoloViews.

[maximlt]

There will be at some point an effort to homogenize the menus across HoloViz' sites.