Utilities/Sphinx: Fix link targets for mixed-case command names

When a CMake domain 'command' object is defined by CMakeTransform or the
'cmake:command' directive, generate the link target with a lower-case
name even if the command name is not all lower-case.  This is needed to
make cross-references to the command definition work since the
'cmake:command' role is marked with the 'lowercase' property.
Merge topic 'doc-mixed-case-commands' into next

607b39dc Utilities/Sphinx: Fix link targets for mixed-case command names