add brush tip creation tutorials for new modes.

We have a tutorial for animated brushes, but not yet for the new brush tip modes.

• How to use gradient tipped brushes for dual loading a brush tip with multiple colors.
• How to easily create brush tips for hairy brushes
• How to create brushes for the lightness mode.
• Update old tutorial.

The two new pages can be previewed:

• height map
• gradient map