Issue 13: Update GitHub actions

[athowes]

Background

Sam writes that we likely want GitHub actions to:

Task	Completion status
Build the pkgdown	Done?
Do a R CMD CHECK against multiple OS's*	Done?
Lint the package using lintr	Done?: it definitely checks code, but perhaps it should be making PRs giving lint changes? I don't know
Code coverage with codecov	Maybe done?
Manage the deployment of our README and the use of the all contributors bot	Done (#60)
Check our cmdstanr code works with the latest version of cmdstanr (optional / stretch goal)	Moved to own issue #104
Benchmarking using touchstone (also a stretch goal / its own issue).	Moved to own issue #103

*if we have things in our tests we don't expect to run on CRAN then we might also way two versions of this [i.e with and without those tests being skipped]

Also noting that:

1. All of these are implemented for epinowcast and so in the first instance we can mostly just copy them across and read through them to check there is nothing specific to epinowcast in them.
2. That done we can change our branch protection rules for main to start requiring some of them be be successful prior to merging
3. For things like linting and R CMD CHECK currently we expect those to fail but a focus early in the development should be to get the code base (via cutting and documentation etc) to a place where those both pass. This should really be done ahead of adding in new features, making interface changes, or adding lots of tests in my view. The other critical CI we need in place before adding tests is code coverage as that will help us when building out the testing suite.

Changes

So far the changes made have been:

• Remove mentions of the master and develop branches (we are not using these)

I have also written where I think things are completed already in the table above.

In doing so, this PR closes #13.

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]

LGTM