Update demand flexibility documentation

### Describe the workflow you want to enable

We need to update (or maybe even create for the first time) documentation that guides users in how to enable demand flexibility in a scenario.

### Describe your proposed implementation, if applicable

We need to make sure that the following documentation is included:
- Show users how to use the different functions needed to enable demand flexibility in a scenario.
- Discuss how the DOE flexibility data is created (or at least point to a different reference of it) and clarify the shortcomings of that data set (i.e., it captures the **technical potential** of demand flexibility as of about a decade ago and does not necessarily capture realistic human behavior).
- Discuss the workflow that we need to make sure is implemented in `PowerSimData` (see #705), which is already implemented in `REISE.jl` (i.e., if flexibility profiles are included, then those are used; if flexibility profiles are not included, then the DOE flexibility profiles are used).

This list is not necessarily comprehensive, and could certainly be added to.