docs: Makefile: Sort Documentation targets case-insensitively in make help

docs: Makefile: Sort Documentation targets case-insensitively in make help

Avoid case-sensitive sorting when listing Documentation targets in make help.
Previously, targets like PCI and RCU appeared ahead of others due to uppercase
names.

Normalize casing during _SPHINXDIRS generation to ensure consistent and
intuitive ordering.

Fixes: 965fc39f7393 ("Documentation: sort _SPHINXDIRS for 'make help'")
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bhanu Seshu Kumar Valluri <bhanuseshukumar@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251104061723.16629-1-bhanuseshukumar@gmail.com>