self-assessment: add Confidential Containers

[mythi]

Add self-assessment of Confidential Containers project (CNCF Sandbox).

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[JustinCappos]

It's a little short / terse in places, but this can be addressed if / when they go for a joint assessment.

[mythi]

It's a little short / terse in places, but this can be addressed if / when they go for a joint assessment.

@JustinCappos thanks for the quick review and approval! I wanted to clarify that I used links to your website docs on purpose because that's where we maintain most of the content these assessments are looking to see. This makes the maintenance easier.