Fix versions.json links for latest sphinx

[uri-granta]

Related issue(s)/PRs: #734

Summary

Unpinning the version of sphinx theme used to generate the docs seems to have broken the links between versions (using the dropdown menu on the topright). Previously the links were absolute (always going to e.g. https://secondmind-labs.github.io/trieste/1.2.0/index.html), but now they're relative, appending the appropriate document path. This is a good thing, but we need to fix and deploy versions.json to get this to work.

Fully backwards compatible: yes

PR checklist

• [x] The quality checks are all passing
• [x] The bug case / new feature is covered by tests
• [ ] Any new features are well-documented (in docstrings or notebooks)

[uri-granta]

Was the issue that the new version of sphinx was appending paths at the end of ../index.html?

Yes. It used to just use the links as they were, but at some point it started appending the relative path to them.