Deprecate MOCK in favor of SKIP

[bveeramani]

Problem

sphinx.ext.doctest renders testcode and doctest blocks in our documentation. While rendering documentation, the extension complains about our custom MOCK option:

/ray/doc/source/data/iterating-over-data.rst:72: WARNING: 'MOCK' is not a valid option.

This wasn't caught before because warnings were accidentally disabled by ray-project/ray#32741.

Solution

Make SKIP behave like MOCK.

SKIP is a valid option. It tells Sphinx to completely skip an example. To avoid warnings, this PR changes the behavior of SKIP so that it runs example code but skips the output.

This won't cause any problems:

1. We don't use SKIP anywhere, so this wouldn't break anything. `
2. SKIP is functionally equivalent to skipif: True, so we can still completely skip examples if we want to.