chore: Add form page examples

[alimpens]

Describe the pull request

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What

It adds some form page examples.

Why

To show how to set up form pages.

How

Typing

Checklist

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Additional notes

• Shouldn't withOneQuestion have an 'other' option? Do these 3 categories cover everything?
• Does 'continue later' make sense for a form flow with 4-6 pages?
• In the designs a Select is used. This is not recommended by gov.uk, because they are hard to use. I've used a Radio group here.
• The design uses a Select for the country code when asking for a phone number. Gov.uk recommends not doing that, but simply allowing users to enter their phone number in a format that's familiar to them. If international numbers are an issue, they recommend adding a description.
• The design uses a 'Previous'-button after the 'Next'-button. Gov.uk recommends placing a back link before the form. NLDS recommends aligning buttons to the start of the line. And Adam Silver explains why using multiple submit buttons in a form is a bad idea. So I've followed the gov.uk recommendation here.

[dlnr]

Since it's a draft I won't go into the code, which looks fine, but I noticed the PageHeader is only showing the menu on the landingpage.

[alimpens]

Since it's a draft I won't go into the code, which looks fine, but I noticed the PageHeader is only showing the menu on the landingpage.

Yeah, that's intentional

[VincentSmedinga]

Great work. So much intricacies here, but the examples are still clearly structured, well-documented, and, of course, a joy to use :)