add --pull flag to sync-docs, pull existing repo if nec

[Rich-Harris]

This improves the sync script a bit — it allows you to specify a -p or --pull flag which pulls from the remote (or clones the repo if necessary), and also specify which packages we care about:

pnpm sync-docs          # sync everything
pnpm sync-docs -w       # sync everything and watch for changes

pnpm sync-docs cli      # just sync `cli` docs
pnpm sync-docs cli kit  # sync `cli` and `kit` docs

pnpm sync-docs -pw kit  # pull or clone `kit` docs, sync, and watch for changes

This is groundwork for setting up an automated workflow