Initial revision of SLSA blog post.

[bendory]

Changes

Added SLSA blog post.

This blog initially appeared at https://opensource.googleblog.com/2023/03/getting-to-slsa-level-2-with-tekton-and-tekton-chains.html

Note: do not merge this PR until the corresponding example has been merged: https://github.com/tektoncd/chains/pull/775

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These are the criteria that every PR should meet, please check them off as you review them:

• [N/A] Includes docs (if user facing)
• [x] Commit messages follow commit message best practices

[bendory]

Preview is now available at: https://deploy-preview-537--tekton.netlify.app/blog/2023/04/17/getting-to-slsa-level-2-with-tekton-and-tekton-chains/

[bendory]

/do-not-merge

confirm that this PR merges first: https://github.com/tektoncd/chains/pull/775

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: geriom

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/website/blob/main/OWNERS)~~ [geriom] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[abayer]

/lgtm