Adds develop trigger to pkgdown action

Pull Request

Checklist:

[ ] I have added new dependencies in DESCRIPTION
[ ] I have bumped the version number in DESCRIPTION
[ ] I have updated the NEWS.md
[ ] I have added unit tests using testthat
[ ] I have checked that the package can install locally using devtools::install_local("<path-to-epireview>")
[ ] I have added documentation where required
[ ] My documentation includes examples of how to use the function
[ ] If necessary, I have updated priority-pathogens

mrc-ide / epireview