Publish old versions of docs

[alexdewar]

Description

Please include a summary of the change and which issue is fixed (if any). Please also
include relevant motivation and context. List any dependencies that are required for
this change.

Closes #1072.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.30%. Comparing base (93cfd18) to head (92cf49d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1178   +/-   ##
=======================================
  Coverage   87.30%   87.30%           
=======================================
  Files          57       57           
  Lines        7706     7706           
  Branches     7706     7706           
=======================================
  Hits         6728     6728           
  Misses        673      673           
  Partials      305      305           

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[alexdewar]

Ok, so I deployed from this branch just to check that it worked (and it did!), but now GitHub is not letting me re-run the workflow for main. I guess it's a service error. I'm logging off now, but I'll try again tomorrow.