Move gh-pages site to the docs/ folder

docs/documentation/advanced-features/validation-of-variables/index.markdown

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+---
+layout: default
+title: Validation of variables
+---
+
+To validate a variable, each time before it is set, define a validation:
+
+```ruby
+validate :some_key do |key, value|
+  if value.length < 5
+    raise Capistrano::ValidationError, "Length of #{key} is too short!"
+  end
+end
+```
+
+Multiple validations can be assigned to a single key. Validations will be executed in the order of registration.