Update panel component guidance for v3 release

[amyhupe]

What

Update the panel component guidance to coincide with the v3 release to ensure it's factually correct.

Why

Some of the guidance will no longer be correct once we've made the release.

Who

A developer, with Amy / Mark to review any changes

Further detail

This sentence: "If you add extra content to the panel, to meet colour contrast ratio requirements use a font size of govuk-body-l for normal weight and govuk-body for bold." is what will need updating.

Done when

• Someone has been assigned to the work
• Changes have been made
• Updates checked and approved by Amy / Mark
• Changes published