[DOCS] add typedef ShaderDefinition and document Shader.definition

[aidinabedi]

Description

This PR takes the type info from the parameter definition in Shader's constructor, and makes it into its own typedef called ShaderDefinition, so that it can be used to also document the property Shader.definition

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[mvaligursky]

It'm not sure about exposing definition as the public API. Two concerns:

1. It seems the user can modify it's content, but in reality it cannot.
2. The content of it is not the same as what the user passed in. As an example we process shaders and replace the originally supplied shaders. And so we cannot provide guarantees as of what it contains.

What's the reason you need definition exposed?