Update UG link in README to link to stable version

Describe the Task

Currently the README links to the "latest" version of the Users Guide on RTD. But we are moving away from using the term "latest". The link is still active and is built from main branch. But the latest tag is hidden on RTD to avoid too much confusion. Need to update the README to link to stable version so we can deactivate latest and cease using it.

Time Estimate

1 hour

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[ ] Complete the issue definition above, including the Time Estimate.
[ ] Fork this repository or create a branch of develop. Branch name: feature_<Issue Number>/<Description>
[ ] Complete the development and test your changes.
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NCAR / wrfcloud

Update UG link in README to link to stable version #184