Removed user retirement docs and add redirect to its new location

[farhan]

Ticket: openedx/edx-platform#34276

Removed user retirement docs and add redirect to its new location

Testing

• Ran ./run_tests.sh without warnings or errors

HTML Version (optional)

• Build an RTD draft for your branch and add a link here

Sandbox (optional)

• Point to or build a sandbox for the software change and add a link here

Post-review

• Add a comment with the description of this change or link this PR to the next release notes task.
• Squash commits

Test Result:

• Docs tested by generating the html files using following command

pip install -r shared/tools.txt
./develop.sh en_us/install_operations/

• Redirection from the old url is working fine

[sarina]

Is there a reason that the pages aren't simply being totally deleted and a redirect set up to the new site?

[feanil]

@farhan you might not be aware, but we can just delete the RST files and replace them with redirects here: https://github.com/openedx/edx-documentation/blob/master/en_us/shared/conf.py#L9