docs: recommend Sifive August 2020 toolchain

[danc86]

Eventually we'll just use conda but in the meantime, update it.

[danc86]

Well, the docs build fails but the error is:

Exception occurred:
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/docutils/writers/html5_polyglot/__init__.py", line 445, in section_title_tags
    if (ids and self.settings.section_self_link
AttributeError: 'Values' object has no attribute 'section_self_link'

which according to sphinx-doc/sphinx#9825 seems to be because we are wrongly using docutils 0.18. So I guess it's probably broken for us on main too, not related to this PR.

Sphinx wants docutils < 0.18 because it seems docutils made some incompatible changes. However we have a somewhat older Sphinx and possibly it does not express the docutils < 0.18 version constraint so pip is giving us this incompatible mixture.

[danc86]

PR #340 should fix the doc build failure in CI.

[tcal-x]

Hi @danc86 , #340 is merged, so can you rebase this to see if it now passes?

[danc86]

Looks like this is all good now!