feat: added image scanning docs

[tomhobson]

Description

This is a draft for the basic of image scanning.

I found this pending, but haven't had time to pick it up to complete it yet.

It would also be nice to include some of the tekton chains bits as well as the extra learnings that we have from trivy now.

Fixes # (issue)

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• I have mentioned the appropriate type(scope), as referenced here in the commit message and PR title for the semantic checks to pass.
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• [ x] Any dependent changes have already been merged.

[tomhobson]

/hold

not ready yet, should have opened as a draft

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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No Coverage information
0.0% Duplication

[jenkins-x-bot]

@tomhobson: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pr	705bb72	link	/test pr
preview	705bb72	link	/test preview

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