Update validation report user guide & tweak model documentation landing page

[nrichers]

Internal Notes for Reviewers

This PR:

• Expands our validation report user guide section, esp. around assessing compliance, linking evidence & findings.
• Adapts the model documentation user guide to follow the same mini landing page design from the validation report section.
• Edits the short descriptions of child topics in each section to work better with the landing page format
• Lots of other smaller edits & tweaks after testing

Major output changes

New landing page for

External Release Notes

The user guides section for working with model documentation and preparing validation reports have been expanded with new landing pages, better procedural task steps, and more contextual information.

Learn more:

• Working with model documentation
• Preparing validation reports

[shortcut-integration]

This pull request has been linked to Shortcut Story #3329: Author guide/link-to-evidence.qmd.

[github-actions]

Pull requests must include at least one of the required labels: internal, highlight, enhancement, bug, deprecation, documentation. Except for internal, pull requests must also include a description in the release notes section.

[github-actions]

Pull requests must include at least one of the required labels: internal, highlight, enhancement, bug, deprecation, documentation. Except for internal, pull requests must also include a description in the release notes section.

[github-actions]

Pull requests must include a description in the release notes section.

[nrichers]

We need this PR live for our Zopa session tomorrow, merging as is.