Docs: docs/how-to-authenticate-ads-api.md restructured
Docs: docs/how-to-authenticate-ads-api.md restructured

Change-Id: I347852c28a027d2100df1d27586e3edf84e08d0c

google/ads-api-report-fetcher,google/ads-api-report-fetcher,google/ads-api-report-fetcher,google/ads-api-report-fetcher