Add Design Patterns in Developer guides (#76)

* developer-guides: Add design-patterns to Developer Guides

* developer-guides: Split design-patterns into 4 pages

* developer-guides: Remove reStructuredText related warnings

* developer-guides: Add image as relative link in design-patterns

Author: sudhanshuv1

source/developer-guides/assets/design-patterns.png

@@ -0,0 +1,83 @@
+Design Patterns
+================
+
+
+.. image:: assets/design-patterns.png
+   :alt: Design Patterns For Humans
+
+--------------
+
+.. raw:: html
+
+   <p align="center">
+
+🎉 Ultra-simplified explanation to design patterns! 🎉
+
+.. raw:: html
+
+   </p>
+
+.. raw:: html
+
+   <p align="center">
+
+A topic that can easily make anyone’s mind wobble. Here I try to make
+them stick in to your mind (and maybe mine) by explaining them in the
+simplest way possible.
+
+.. raw:: html
+
+   </p>
+
+--------------
+
+🚀 Introduction
+----------------
+
+Design patterns are solutions to recurring problems; **guidelines on how
+to tackle certain problems**. They are not classes, packages or
+libraries that you can plug into your application and wait for the magic
+to happen. These are, rather, guidelines on how to tackle certain
+problems in certain situations.
+
+   Design patterns are solutions to recurring problems; guidelines on
+   how to tackle certain problems
+
+Wikipedia describes them as
+
+   In software engineering, a software design pattern is a general
+   reusable solution to a commonly occurring problem within a given
+   context in software design. It is not a finished design that can be
+   transformed directly into source or machine code. It is a description
+   or template for how to solve a problem that can be used in many
+   different situations.
+
+⚠️ Be Careful
+-------------
+
+-  Design patterns are not a silver bullet to all your problems.
+-  Do not try to force them; bad things are supposed to happen, if done
+   so. Keep in mind that design patterns are solutions **to** problems,
+   not solutions **finding** problems; so don’t overthink.
+-  If used in a correct place in a correct manner, they can prove to be
+   a savior; or else they can result in a horrible mess of a code.
+
+..
+
+   Also note that the code samples below are in Vala, however this
+   shouldn’t stop you because the concepts are same anyways. Plus the
+   **support for other languages is underway**.
+
+Types of Design Patterns
+------------------------
+
+-  :doc:`Creational </developer-guides/design-patterns/01-00-creational-design-patterns>`
+-  :doc:`Structural </developer-guides/design-patterns/02-00-structural-design-patterns>`
+-  :doc:`Behavioral </developer-guides/design-patterns/03-00-behavioral-design-patterns>`
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+   :numbered:
+
+   design-patterns/*