Rework documentation only build decision including GCC 10.3.0/11.1.0

[ptheywood]

Documentation only build logic is changed as it is now more than just old / missing cuda.

• If GCC is too old for FLAME GPU, but new enough for CUDA block it. (< 6)
• If GCC is too old for the test suite, just block the test suite (< 7)
• If CUDA is not available (or implicitly if the host compiler is explicitly blocked by CUDA), do a docs build.
• If CUDA is too old for FLAME GPU, do a docs build (< 10)
• If GCC is the host compiler, and GCC is version 10.3.0 or 11.1.0, docs only.
  • This is due to a bug in GCC's <chrono>, which has already been patched for later versions (10.4, 11.2).
  • Closes GCC 10.3.0 internal compiler error #575