build: default to wombat proxy for publishing

[josephperrott]

If no --registry argument is provided when calling to publish
use the Wombat proxy. Additionally, updates the release process
documentation to instruct usage of the Wombat proxy.

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "ci/angular: size" is failing
     status "ci/circleci: e2e-cli" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[alan-agius4]

One small NIT

LGTM.

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