[TF-M] [Doc]Update sphinx to 2.0.1

Hi,

To better maintain tf-m documentation, we plan to enable 'turn warnings into errors' in documentation HTML generation. The patchhttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10424 is on review now.

One note is that sphinx should be updated to 2.0.1, otherwise there will be some 'duplicate label' warnings, which will fail documentation generation.

Best Regards, Summer