Docs: improve the icon categories

[brandyscarney]

I think we need to improve the Icon categories, something like this:

• Directional Icons
• Sport Icons
• Currency Icons
• Logo Icons

etc.

[peterennis]

@brandyscarney

• Custom Icons

Document the process to be able to add personal/client branding.

• Build should work on Linux AND Windows.
• Option should exist that does not require the inclusion of ionicons website during build.