Documentation/README.MSYS2.md:
Provide an overview of the different MSYS2 environments and what
they provide from the perspective of installing and using Mercury.
Describe how to install and use Mercury in the MSYS environment.
Describe how to install Mercury in the CLANG64 environment.
Extend the coverage of running Mercury at the Windows Command Prompt, to
cover Mercury compilers built in the CLANG64 and MINGW32 environments.
Describe installation prefix path problems in more detail.
Lots of minor improvements.
Documentation/README.MSYS2.md:
Add a section on using Mercury compilers created in the MSYS2 environment
at the Windows Command Prompt. As of rotd-2025-12-31, this now works
out of the box, without requiring users to edit Mercury.config.
Fix some markup errors.
Minor adjustments to other sections.
Documentation/README.MinGW:
As above.
Documentation/README.MS-Windows.md:
Conform to the above change. Delete references to the original
MSYS platform.
