README updates: suggest cmdstanr + nowcasting course resource

[jamesmbaazam]

Description

This PR updates the README with new text to fix #706 and resolve #704.

The PR adds a paragraph to the README to surface the availability of approximate methods alongside MCMC and suggests installing cmdstanr if users want to use Pathfinder and Laplace. Additionally, the PR adds a line to the resources section to link to the nowcasting course.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

Nice. LGTM