Small fixes for 0.9.4 release

[murrayrm]

This PR implements a number of small fixes for issues tagged for resolution in release 0.9.4:

• Updated installation documentation to recommend conda installation first, then pip (resolves Change installation instructions to recommend Conda, warn about compiling Slycot #850)
• Added more documentation about Nyquist plot scaling (resolves Incorrect nyquist plot #830)
• Deprecated issys() (resolves ctrlutil.issys() is confusing (tests for LTI) #876)
• Changed short package name for control.optimal from opt to obc consistently throughout documentation.
• Fixed sphinx processing of version to allow proper links to source code (resolves Readthedocs points to wrong version of source code for .postN release #837).
• Added numpy 1.24.2 to list of exceptions for flatness test on linux using Generic BLAS (resolves Numpy 1.24.2 breaks OS/BLAS test matrix #866).
• Additional updates to docstrings and rst files as I went through the above items.

[coveralls]

Coverage: 94.535% (+0.001%) from 94.533% when pulling 1d7480a on murrayrm:smallfixes-26Mar2023 into 0422c82 on python-control:main.

[sawyerbfuller]

LGTM.