Hide 'GPU Computing' tutorial behind a feature flag

[jeff-phillips-18]

Fixes:
Jira: https://issues.redhat.com/browse/RHODS-1918

Analysis / Root cause:
GPU selection is not available so the tutorials about GPU computing (i.e., GPU Computing in Python with Numba) should be hidden in the "Resources" tab of RHODS

Solution Description:
Mark the GPU Computing in Python with Numba tutorial as requiring a feature flag of gpu-computing which is set to false.

• - [x] The commits are squashed in a cohesive manner and have meaningful messages.
• For commits that came from upstream, [UPSTREAM] has been prepended to the commit message
• JIRA link(s): https://issues.redhat.com/browse/RHODS-1918
• The Jira story is acked
• An entry has been added to the latest build document in Build Announcements Folder.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious)

[cfchase]

LGTM