docs/conf.py: Update to handle RTD changes

[kd-ods]

Read The Docs has changed the way it works. Rather than automatically, and silently, messing with sphinx config files to add certain things, it expects certain changes to the config file for everything to work.

Overview

What does this pull request do?

Updates docs/conf.py

How can a reviewer test or examine your changes?

Check the build is working at https://standard.openownership.org/en/rtd_config_update/standard/modelling/index.html

Who is best placed to review it?

Ed

Closes #

Translations

n/a

Documentation & Release

• I've checked that the documentation builds without failing. See the Handbook
• n/a I've thought about how and when this needs to be released. See the Handbook
• n/a I've updated the changelog, if necessary. See the Handbook
• n/a I've updated reference.rst to reflect any changes to schema structure or naming.
• n/a I've updated the BODS validation specification if necessary. See the Handbook
• n/a I've added or edited any other related documentation. See the Handbook

[kd-ods]

@radix0000 - could you please OK this PR so I can merge it. Don't worry about the failing link check. I'm dealing with that on the docs update PR.