Add epinowcast/actions/install-cmdstan + pgdown as env object

[seabbs]

Description

This PR is not linked to an issue. It overhauls the current CI to better match practice in epinowcast (mostly this is using our cmdstan action which has caching to speed up builds of CI) in order to begin looking at the issue with the pkgdown docs. With the same aim in mind it also adds saving the built website as an enviroment object (which is not standard epinowcast practice but should imo).

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.

[seabbs]

Its a bit in the weeds and ideally it would post a comment with a link or something but if you click on the pkgdown workflow and then on summary you should see something called website which is the built website.

Note that pkgdown is now working as expected despite me not having really touched it. Potentially this was a transient pkgdown issue which we see due to using the dev version (which we could think about if we really need but these kind of passing issues are kind of a feature of CI build systems sadly)

[seabbs]

I triggered a build on main and yes it looks like the problem has cleared itself up≥

[athowes]

I've not used these functions before. If you have a chance, putting a brief response to my questions / clarifications would be helpful for me to learn what's going on. Other than that, happy to merge.