Basic Emitter/Visitor docs (#892)

Author: HurricanKai

documentation/for-contributors/generators/emitter/visitor.md

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+# Visitor
+
+To Transform our internal symbol layer to C# code a visitor (`CSharpEmitter.Visitor`) is used.
+
+## State
+
+To communicate produced syntax nodes with the levels above the current one the state variables are used.
+Therefore its important to enshure that this state is clear at all times. (Using `AssertClearState` & `ClearState`)
+
+## Depth First
+
+In general we try to push symbols down as far as possible to avoid code duplication and increase testable surface area.
+The best example of this that instead of creating identifier tokens / identifier name syntax nodes inline (which would be very easy) we visit the identifier, letting the `VisitIdentifier` function create those, and then take what we need, using a pattern like
+
+```cs
+VisitIdentifier(structSymbol.Identifier);
+if (_syntaxToken is not { } identifierToken)
+    throw new InvalidOperationException("Some Identifier was not visited correctly");
+ClearState();
+```
+
+when a token is required, or
+
+```cs
+VisitIdentifier(fieldSymbol.Type.Identifier);
+if (_syntax is not IdentifierNameSyntax typeIdentifierSyntax)
+    throw new InvalidOperationException("Some Identifier was not visited correctly");
+ClearState();
+```
+
+when a syntax node is required.
+
+## Trivia
+
+The visitor is intended to produce "valid" code, that means it can be formatted using .ToFullString and the resulting code is valid and can be parsed back by roslyn.
+In addition we try to avoid elastic trivia wherever possible. For performance reasons we don't want to normalize our trivia / syntax internally, but elastic trivia would be thrown away when doing so, therefore we try to avoid generating elastic trivia in the first place.
+Integration tests should test for elastic trivia, although their primary purpose is to check the resulting formatted string.