Add placeholder referring to "repository-data/site" module

In order for Essentials plugins to install YAML resources into a brXM project, Essentials' Plugin SDK API provides placeholders for the repository-data/application and repository-data/development modules. 13.0 adds a repository-data/site submodule. This module should also have a set of placeholders.

Since the application and development modules pertain to the "platform" rather than a "site", they should be renamed to "platformApplication..." and "platformDevelopment..." respectively, for better clarity.