Change PYANSYS_PYPI_PRIVATE_PAT to PYANSYS_PYPI_PRIVATE_READ_PAT in docs

[klmcadams]

Description of the modifications

Change the PAT in the docs to the READ_PAT because of this:

The PAT with "write" access is only accessible through CI/CD, forcing devs to release by workflows, which is a good practice.

Useful links and references

https://dev.docs.pyansys.com/how-to/releasing.html#artifact-download