Add GitHub actions

[cjrace]

Added two GitHub actions files:

1. Standard package checks

This was done using usethis::use_github_action("check-standard"), and adds R CMD checks against a number of operating systems for the package.

This added a status badge to the readme too as a bonus 🦡

Have set to run against all pushes and PRs to main/master (default) and also any PRs to our working branches (development / update-to-gds-v5.4).

More information on these checks can be found at - https://r-pkgs.org/r-cmd-check.html. Effectively it's similar to running devtools::check() locally.

2. Build pkgdown site

Realised the current docs aren't hosted anywhere, so added a workflow that we can test when we do merge to main to add in the documentation onto a github pages site.

You can preview the site using devtools::build_site() though be aware it generates a bunch of updates to files in the docs/ folder. Try to avoid committing any of these, as ultimately we want to ignore them. I've raised #80 for me to remember to check this and do clean up of the Git history afterwards, which will stop this happening in the long run.

[sarahmwong]

I'll assign the rest of the issues in our checklist once you've merged in :)