[BSv5] Docsy variables cleanup and BS version test

[chalin]

• Checks that BSv5 is being used by a project, otherwise an error is reported by the SASS compiler.
• Cleans out _variables.scss:
  • Factors out forward declarations of BS SCSS variables into their own file, so that it is clear that they are forward declarations: _variables_forward.scss. Eventually, the content of this file will be auto-generated to avoid definition mismatches.
  • In the process of the refactoring previously mentioned, I dropped unnecessary redeclarations of BS SCSS variables (of the same value).
• Switches to using the "BSv5-way" of customizing, e.g.,:
  • Now making use of $link-shade-percentage, rather than overriding $link-hover-color
• Switches to an official BSv5 $blue-* color variable for the link color rather than use a custom adjusted color.
• Marks some Docsy variable declarations as candidates for namespacing and/or moving into the UG -- I'll open an issue to follow up on this.

[LisaFC]

Looks great!

[chalin]

For the record, this PR introduced an error, that was fixed via:

• Allow link-color to be set by projects #1631