Move dev docs

[AdrianDAlessandro]

Description

This PR moves the development documentation so that is no longer the top-level documentation but is instead alongside the other releases.

There are no longer any top-level docs at all, instead, there is a redirect that takes the user to the most recent (stable) version of the docs when using the link in the readme (https://energysystemsmodellinglab.github.io/MUSE2/).

I have also removed the extra release layer in the url path because it is no longer necessary given the top level docs don't exist at all. So the development docs will now live at https://energysystemsmodellinglab.github.io/MUSE2/dev/ (and replace dev with any version string for other versions)

Fixes #1193
Fixes #1194

Note: Requires #1356 to be merged first

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

[AdrianDAlessandro]

I am aware that the link checker tests are failing. They will continue to fail until this branch is merged because there are absolute urls that will not exist until after this version of the docs is deployed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (33466f6) to head (5ea6830).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           docs-version-selector    #1366   +/-   ##
======================================================
  Coverage                  89.75%   89.75%           
======================================================
  Files                         58       58           
  Lines                       8529     8529           
  Branches                    8529     8529           
======================================================
  Hits                        7655     7655           
  Misses                       561      561           
  Partials                     313      313           

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