BigQuery: Fix anchors in URLs in external_config module

[plamut]

In external_config.py, many links in docstrings point to non-existing HTML anchors, making them less convenient to open the page at the exact property/object the link points to.

Example:
The link to the compression property

https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#externalDataConfiguration.compression

should actually be

https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#ExternalDataConfiguration.FIELDS.compression