doc(CB-12770): revise security documentation

[erisu]

Motivation and Context

Closes #703

Would like additional feedback & suggestions on the content. The original content was written over 8 years ago and it is possible some content is no longer needed.

Description

The content of this PR is:

• From CB-12770: revise security documentation #703
• Rebased with main branch
• Applied suggestions from CB-12770: revise security documentation #703
• Some refactoring on working, content placement, etc..
• Completed the Encrypted Storage TBD section with small generic content.

Testing

n/a

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[breautek]

lgtm