Allow working out of the box on CI jobs with Github Actions

Currently, if you run on Travis and leave `addon-docs.js` empty it will just work because it [falls back to `process.env.TRAVIS_BRANCH`](https://github.com/ember-learn/ember-cli-addon-docs/blob/731830a208e6a36562a4639867439649720a032a/lib/config.js#L32) to grab the correct `getVersionPath`

We should allow the same fallback behavior for Github Actions.

If, when using Github Actions, you don't explicitly set you `getVersionPath()` or `ADDON_DOCS_VERSION_PATH` you would get error:

```
Run yarn ember deploy production
yarn run v1.19.2
$ /home/runner/work/ember-transformicons/ember-transformicons/node_modules/.bin/ember deploy production
no addon-docs version path configured; skipping deploy
undefinedPipeline aborted
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
##[error]Process completed with exit code 1.
```