Update style guide to denote correct case style

[glemieux]

Description

The current documentation notes that we use CamelCase, but what we actually use by definition is PascalCase (where we to use the former we would use camelCase).

Suggested fix

Correct the style guide to note that we use PascalCase. Also update the guide to note that the use of underscores in variable names is snake_case. It looks like we also use camelCase for derived type naming (e.g. currentSite) which seems like a good distinction to keep between objects and variables.