Issue 64: In depth model vignette

[athowes]

Description

Status: work in progress! Ready for writing again when it's a priority.

This is a draft PR for issue #64.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[athowes]

Updates based on #70:

• Now want to re-explain how the model works with current implementation
• Should be easier to explain the parts than before

[athowes]

Once #179 is merged this can be moved back to being worked on.

[athowes]

I am closing this PR as this vignette is a "nice to have" over a requirement for 0.1.0 hence I won't be working on it soon.