Add github action to knit Rmd

[stefpiatek]

• Knit and commit any changes to Rmd files in CI.
• Added renv to make my life easier
• Trivial change to Rmd to check that the action works as expected

[stefpiatek]

Hmm frustratingly I can't install arrow locally because of a dependency issue, which means I can't test if this Rmd should knit locally or not.

Failing in CI with ! object 'gender_concept_name' not found

[andysouth]

Good persistence :-)