RI::Paths

Encapsulate all the strangeness to do with finding out where to find RDoc files

We basically deal with three directories:

1. The ‘system’ documentation directory, which holds the documentation distributed with Ruby, and which is managed by the Ruby install process
2. The ‘site’ directory, which contains site-wide documentation added locally.
3. The ‘user’ documentation directory, stored under the user’s own home directory.

There’s contention about all this, but for now: