Document stance on BootstrapVue

There was a proposal to vendor BootstrapVue (BSV) in order to make invasive upstream changes. That was ultimately closed/abandoned, but we still need to formalise/document our stance on BootstrapVue. We need answers to questions like:

• Should new components be based on BSV?
• What should we do with existing thin wrappers around BSV components?
• What about complex components like GlTable?
• What about Bootstrap?
• What should we do with &574 (closed) and child issues?

And so on.