Beginner Repository Build Documentation

Add beginner/introductory focused instructions for installing any necessary libraries and building the contents of the OSL repo. This would document all the steps necessary to successfully build all the OSL repo contents.

I have a Siggraph 2025 OSL Lab submission that could be re-designed to cover this exact material, which could then be added to the repository. The lab materials could also cover the use of osltoy and/or testrender as tools for .osl shader testing and/or development.

Start by reviewing the OSL repository's current build process and the proposed Siggraph 2025 OSL Lab materials. Inventory the libraries and steps needed to build all repository contents, and decide whether osltoy and testrender are included. Done means a beginner-focused guide covers the complete build successfully and explains the relevant shader-testing tools.

Written by the indexing model from the issue text.