[docs] Try to make it easier to find info about new PM vs legacy PM (#80834)

Seen several beginner questions popping up in discourse about how to
implement and run custom passes. And then it turns out that they are
following the old "Writing an LLVM Pass" guide that describe legacy
passes, and then things are mixed up when they try to run that pass
using opt that nowadays default to the new pass manager.

This is an attempt to make it slightly clearer in the User Guides that
there are two different "Writing an LLVM Pass" pages depending on which
pass manager that should be used. This is done by renaming the legacy
version of "Writing an LLVM Pass" as "Writing an LLVM Pass (legacy PM
version)".
Also reordered the links to put the link to the new pass manager
documentation first.

This patch also moves the warning text that cross references the
description on how to write a pass for legacy/new PM to make sure it
ends up already in the beginning of the descriptions.

Also adding a new warning in the "Running a pass with opt" section of
the legacy PM version of the guide, to inform that those examples are
outdated.