Example showing grdgradient shading outputs with different radiance or normalize values

[weiji14]

Description of the desired feature

The default shading/-I of grdimage is +a-45+nt1+m0, or an azimuth of 45deg, intensity of 1 and radiance of 0. The intensity settings can also be controlled in a finer manner using https://www.pygmt.org/v0.8.0/api/generated/pygmt.grdgradient.html.

To make it more intuitive to understand what different intensity (-1 to +1) and radiance (0 to 1) settings look like, it would be good to have an example showing the differences. One idea is to have a gallery example plotting some DEM with the different combinations in a grid subplot.

	radiance=0	radiance=0.5	radiance=1
normalize=-1
normalize=0
normalize=+1

Alternatively, we could also create a full tutorial that walks through some of the advanced features of grdgradient.

This was an idea suggested by @JingHuiTong during the AGU 2022 Fall Meeting.

Extending from #1425.

Are you willing to help implement and maintain this feature?

Maybe