Docs: Create docs outlining how to extend blueprints from blueprints

Common author request:
See branch https://github.com/aws/bex-blueprints/tree/extention

Start by reviewing the extention branch linked in the issue to understand the blueprint extension work it demonstrates. Document how authors extend blueprints from other blueprints, and consider the work done when a newcomer can follow the resulting guidance.

Written by the indexing model from the issue text.