Switch to `main` version in URLs

Once all content repos have switched to using a `main` branch instead of `master`, we should publish docs under URLs containing `main`. Right now, we use `mapMasterToMain` and `mapMainToMaster` in `conf.yaml` to ensure mappings between different repos that use different branches. In effect, we are still publishing a **version** called "master" even when building from a **branch** called `main`.