Fix building docs on RTD

[p-j-smith]

Changes made in this Pull Request:

• add rust as a build tool to the RTD config
• add maturin as a docs dependency
• use stable rust toolchain for building docs

PR Checklist

• Tests added and passing?
• Docs added and building?
• CHANGELOG updated?
• AUTHORS updated if necessary?
• Issue raised and referenced?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (af97173) 98.88% compared to head (0834b99) 98.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #139   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files          19       19           
  Lines        1168     1168           
  Branches      183      183           
=======================================
  Hits         1155     1155           
  Misses         13       13           

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