[GH-2793] Exclude examples from documented methods

Example code in various providers directories is getting picked up by
Yard when documenting such that a number of global constants and helpers
are appearing to be part of fog.

This excludes them using `.yardopts`

It also excludes generated documentation from provider subdirectories
from git.

.gitignore

@@ -26,3 +26,5 @@ spec/credentials.yml
 vendor/*
 tags
 tests/digitalocean/fixtures/
+
+providers/*/doc

.yardopts

@@ -0,0 +1 @@
+--exclude 'lib/fog/.*/examples'