Introduce configuration value for static site generator

Problem to solve

Add a configuration value for the type of static site generator used

Proposal

Add a new value to the configuration file called static_site_generator. Possible values: middleman (default), hugo (NOTE: hugo will not be added until it is supported).

Implementation

NOTE: This config value is being implemented as part of #241164 (closed), as the first simple single-supported-value entry in the config file, in order to drive the development of the config file infrastructure.

Related Issues

• relates #241164 (closed) (implemented by this issue)