Add CNCF Sandbox application related docs

[jnummelin]

GTR: General technical review, focusing on "day 0" parts. https://github.com/cncf/toc/blob/main/tags/resources/toc-supporting-guides/general-technical-questions.md

Security self-assessment: Security self-assessment, focusing on the basics. https://tag-security.cncf.io/community/assessments/guide/self-assessment/

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[twz123]

What about moving this to docs/cncf?

[twz123]

s/Github/GitHub/g