docs: prevent build error by rolling back Sphinx version

docs: prevent build error by rolling back Sphinx version

We're hitting a sphinxcontrib-httpdomain vs sphinx issue that was
causing the openapi part of doc builds to error out with:

<openapi>:1:Problem in http domain: field is supposed to use role 'obj', but that role is not in the domain.

See https://github.com/sphinx-contrib/httpdomain/pull/51

Until it's fixed, hold back the Sphinx version to < 4.1.0

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>