Attempt to make WinUSB install notes clearer

[Kuuuube]

I finally waded through the dependency jungle and succeeded in getting this thing running to test my changes... bundler does not come installed by default. The erb gem was removed at some point or something like that so we need to add it explicitly. At least this is what I needed to get it working on Arch.

• WinUSB installation is now a substep.
• The supported tablets list is explained to not be a list of tablets that needs WinUSB but rather a list of tablets that OTD supports. There's been at least two people now who have come to support channels confused about this specifically.
• Added (Usually not required) to the step name

If all of this doesnt make people turn their brains on for a second, I think it's a lost cause.

[gonX]

LGTM.

Re erb dependency addition: it indeed seems necessary as I apparently have that package implicitly installed as a dependency for the Ruby default gems, so I never ran into this library not being available.

While it is in the standard gems list, I don't think we can expect this to be present everywhere? Maybe some Ruby expert can chime in on that part of the change actually being necessary, but regardless this change as it is is an improvement.

Ruby docs does say that default gems are not uninstallable but this doesn't necessarily translate to meaning that the default gems are always available.