Update RTD installation instructions

[hbushouse]

This PR addresses an issue raised in Help Desk ticket INC0196726 where there was no mention of the fact that we are currently limited to installing jwst with python<=3.11. So I've updated all the places where "conda create" is mentioned.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f479fcc) 75.24% compared to head (3523670) 75.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8200   +/-   ##
=======================================
  Coverage   75.24%   75.24%           
=======================================
  Files         470      470           
  Lines       38431    38431           
=======================================
  Hits        28919    28919           
  Misses       9512     9512           

Flag	Coverage Δ		*Carryforward flag
nightly	77.37% <ø> (ø)		Carriedforward from 4cfb4ed

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[jdavies-st]

I note that you are successfully testing under 3.12 now in the 1.13.x tagged versions and on master. Will Python 3.12 eventually be officially supported once one of those 1.13.x releases makes it to pypi?