Insert link into README, instead of nunjucks link

[sofisl]

This template uses nunjucks links which don't render properly in npm: https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/node_mono_repo_library/README.md

(but they do on the README)

We need to actually link the correct links instead of:

{% if metadata['repo']['client_documentation'] %}[client-docs]: {{ metadata['repo']['client_documentation'] }}{% endif %}
{% if metadata['repo']['product_documentation'] %}[product-docs]: {{ metadata['repo']['product_documentation'] }}{% endif %}
[shell_img]: https://gstatic.com/cloudssh/images/open-btn.png
[projects]: https://console.cloud.google.com/project
[billing]: https://support.google.com/cloud/answer/6293499#enable-billing
{% if metadata['repo']['api_id'] %}[enable_api]: https://console.cloud.google.com/flows/enableapi?apiid={{ metadata['repo']['api_id'] }}{% endif %}
[auth]: https://cloud.google.com/docs/authentication/getting-started

See issue for Compute: https://github.com/googleapis/google-cloud-node/pull/4983