[docsite] add advanced/processor-steps section

docsite/source/advanced.html.md

@@ -8,6 +8,7 @@ sections:
   - key-maps
   - rule-ast
   - custom-types
+  - processor-steps
 ---
 
 - [Predicate Logic](/gems/dry-schema/advanced/predicate-logic)

docsite/source/advanced/processor-steps.html.md

@@ -0,0 +1,37 @@
+---
+title: Processor steps
+layout: gem-single
+name: dry-schema
+---
+
+^WARN
+This feature is experimental. It should become stable in version 2.0.0.
+^
+
+Schemas process the input using 4 steps:
+
+1. `key_coercer` - Prepare input hash using a key map
+2. `filter_schema` - Apply pre-coercion filtering rules
+   (optional step, used only when `filter` was used)
+3. `value_coercer` - Apply value coercions based on type specifications
+4. `rule_applier` - Apply rules
+
+It is possible to add `before` or `after` callbacks to theses steps if you wish to customize processing. Let's say you want to remove all keys with `nil` values before coercion is applied:
+
+```ruby
+schema = Dry::Schema.Params do
+  required(:name).value(:string)
+  optional(:age).value(:integer)
+
+  before(:value_coercer) do |result|
+    result.to_h.compact
+  end
+end
+```
+
+Now when the schema is applied, it'll remove all keys with `nil` values before coercions and rules are applied:
+
+```ruby
+schema.(name: "Jane", age: nil)
+# => #<Dry::Schema::Result{:name=>"jane"} errors={}>
+```