Draft documentation for using test fixtures

[36degrees]

Part of #1830

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What

Draft documentation on how to use test fixtures to check that a port uses the same HTML as GOV.UK Frontend.

Why

Doing this will allow us to include draft documentation in a proposal we can share with the community in order to get feedback.

[36degrees]

We'll revisit what we need to do around documenting this once we've put the proposal out and got feedback from the community.

[hannalaakso]

After chatting to @m-green we're going to revisit the format of the release notes as part of this documentation work.

[m-green]

I can't add the docs as a linked PR so here's the links:

• Add docs on testing with HTML fixtures govuk-frontend-docs#74
• Add note for contributors on using hidden:true in yaml files #1922