Repeat VOT validation guidance in token validation section

[ethanmills]

Why

These validation steps are essential to guard against request tampering

What

Update the token validation guidance and emphasise the importance of it. Adapted from https://github.com/govuk-one-login/tech-docs/blob/main/source/integrate-with-integration-environment/prove-users-identity.html.md.erb#L254

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Changelog

If this change is significant (for example, launching a new feature or deprecating a feature), you should update the changelog found at partials/_changelog.erb under the heading 'Documentation updates'.

Confirm

• I have checked if any docs change here also requires updates to other repositories (ADRs / RFCs, README.md, Team Manual, elsewhere in these docs)
• Where there is any overlap I have updated or opened a PR for corresponding changes