Remove old documentation in manual/ folder

The docs are available at https://docs.rubocop.org/rubocop-rspec/ and no
longer at https://rubocop-rspec.readthedocs.io/

Author: bquorning

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Master (Unreleased)
 
+* Move our documentation from rubocop-rspec.readthedocs.io to docs.rubocop.org/rubocop-rspec. ([@bquorning][])
 * Add `RSpec/RepeatedIncludeExample` cop. ([@biinari][])
 * Fix `RSpec/FilePath` when checking a file with a shared example. ([@pirj][])
 * Fix subject nesting detection in `RSpec/LeadingSubject`. ([@pirj][])

Rakefile

@@ -53,11 +53,11 @@ end
 desc 'Confirm documentation is up to date'
 task confirm_documentation: :generate_cops_documentation do
   _, _, _, process =
-    Open3.popen3('git diff --exit-code manual/')
+    Open3.popen3('git diff --exit-code docs/')
 
   unless process.value.success?
     raise 'Please run `rake generate_cops_documentation` ' \
-          'and add manual/ to the commit.'
+          'and add docs/ to the commit.'
   end
 end