Add documentation for geoviews.feature

[mosc9575]

Please extend the documentation for the geoviews.features like gf.land and gf.coastline.

When using the search on the https://geoviews.org/ one result is the link to geoviews.feature.html which is an empty page.

I was running the first example on the start page, and I was confused by the missing border between Germany and Poland.

import geoviews as gv
import geoviews.feature as gf
from cartopy import crs

gv.extension('bokeh')

(gf.ocean * gf.land * gf.coastline * gf.borders).opts(
    'Feature', projection=crs.Geostationary(), height=325
)

For me it was very difficult to find the .options(scale="50m") call for features. With a different scale the border is drawn but I still don't know which arguments are valid and where it is documented.

[hoxbro]

The missing border was fixed by #813