Update alt text guidance, example and image

[calvin-lau-sig7]

Update 'Images' guidance to include:

• when to use alt text
• how to write alt text

This guidance is in coordination with GOV.UK Accessibility team.

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[calvin-lau-sig7]

Very nearly ready to publish!

I've asked @ImranH-GDS to give this a fresh look. Some lovely folks from the GOV.UK Accessibility team are also looking at it.

Here's a direct link to the preview page: https://deploy-preview-2059--govuk-design-system-preview.netlify.app/styles/images/

[calvin-lau-sig7]

Content from PR #2240 is being consolidated into this PR (see 'headings sort' in this doc)

[CharlotteDowns]

Include links to image attribution in gov.uk guidance to encourage use of original content

[calvin-lau-sig7]

@CharlotteDowns Thanks! Really appreciate the proofread. I've added them in and squashed.

[calvin-lau-sig7]

Hi @jbuller, @joelanman, @selfthinker

At long last, we're getting close to being ready to publish an iteration of this guidance. Thanks for your input into this throughout!

We're likely to publish this under an 'experimental' tag in hopes of getting feedback and examples of images. Aiming to make this page and guidance as iterative as we can.