publish "dev" documentation

[aronatkins]

We should publish development versions of the documentation rather than publishing only on CRAN release.

That documentation would be available at https://pkgs.rstudio.com/connectapi/dev/ and the released documentation would remain at https://pkgs.rstudio.com/connectapi/.

For comparison, here is the testthat documentation:

Release: https://testthat.r-lib.org/
Development: https://testthat.r-lib.org/dev/

[aronatkins]

Documentation would have been helpful during discussions in:
rstudio/rsconnect#888
#196