Document global variables

These should be documented on their own page:

https://github.com/LLNL/blt/blob/1720b660fcdc121b68d370444f192be5c3db1ceb/SetupBLT.cmake#L175-L182

There may be more.

Start by reading the global-variable definitions in SetupBLT.cmake at lines 175-182 and inspect nearby entries for additional variables. Create a standalone documentation page covering the identified globals, including any further ones found during the review; done means the relevant variables are documented on that page.

Written by the indexing model from the issue text.