docs: pin sphinx dependency to versions <7.0.0

This commit pins sphinx to versions lower than 7.0.0 to resolve
incompatibilities with older versions of sphinx-rtd-theme.

Change-Id: Iddef62f8a80120cc457c616e8bd737cce3e1f1a1
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
1 file changed