Improve documentation structure

docs/00-intro.md

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+---
+title: Introduction
+order: 1
+---
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 # Introduction
 
 `bakame/http-structured-fields` is a framework-agnostic PHP library that allows you to parse, serialize, 

docs/01-basic-usage.md

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+---
+title: Basic usage
+order: 2
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 # Basic Usage
 
 ## Parsing the Field

docs/02-parsing-serializing.md

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+title: Parsing and Serializing HTTP Fields
+order: 3
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 # Parsing and Serializing HTTP Fields
 
 ## Parsing fields

docs/03-value-types.md

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+title: HTTP Fields value types
+order: 4
+---
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 # Structured Fields Values
 
 ## Value type conversion to PHP

docs/04-item.md

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+---
+title: The Structured Field Item Data Type
+order: 5
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 # The Item Data Type
 
 This is the structure from which you will be able to access the actual field content.

docs/05-containers.md

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+title: The Structured Field containers Data Types
+order: 6
+---
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 # Containers
 
 While building or updating a Bare Item is straightforward, doing the same with the structured field

docs/06-validation.md

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+title: Structured Field validation
+order: 7
+---
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 # Validation
 
 When it comes to make sure about the incoming data the package provides a simple approach around validation,

docs/07-extensions.md

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+title: Interacting with PHP ecosystem
+order: 8
+---
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 # Interacting with PHP ecosystem
 
 All Datatypes expose the `Stringable` interface while it is recommended to use