Add manuscript to git

[nikosbosse]

Description

The manuscript is relatively mature now, so I think it makes sense to add the pdf to the git repository. This PR

• updates .gitignore to do that.
• updates the manuscript by adding a necessary as_forecast().
• shrinks a few images to reduce file size.

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 as follows: issue-number: PR title
• [ ] I have tested my changes locally.
• [ ] I have added or updated unit tests where necessary.
• [ ] I have updated the documentation if required.
• [ ] I have built the package locally and run rebuilt docs using roxygen2.
• [ ] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [ ] 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.

[nikosbosse]

I'm just going to merge this as it isn't related to code changes and I want to continue working on #796. Apologies!