Updating installation instructions for PIP version limitation

[john-science]

What is the change?

Updating the installation docs to explicitly state the PIP version limitation.

Why is the change being made?

ARMI depends on the pyproject.toml feature set, which is not fully supported until pip>=22.1 (which was released in May 2022). So, this version limit is mandatory.

Notes

@albeanth :

1. This is a SCR-like change, because the "User Installation" guide is part of our official documentation.
2. All our downstream projects already explicitly state the pip>=22.1 dependency limit; so no change needs to be made to those projects.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@caiodallaqua I have made your requested improvement to the docs in this PR: #1639 (comment)

[albeanth]

Seems reasonable enough to me. So instead of the stub file @ntouran suggests, we're enforcing a min version of pip. This enforcement aligns with at least one of our internal projects, so no problems on that front.