Document how to write and use sweepers in contributor docs

[melinath]

What kind of contribution is this issue about?

Other (specify in details)

Details

We should document how to write and use sweepers. Currently we have https://github.com/hashicorp/terraform-provider-google/wiki/Developer-Best-Practices#sweepers but that's not accessible to users.

This may make sense to do as part of a broader effort to make sweepers more accessible to users, for example by making them configurable.

References

#16859
#17835
yaqs/6453048037068505088

[SarahFrench]

Data point for this issue: GoogleCloudPlatform/magic-modules#11413 (comment)

[melinath]

Hashicorp's documentation about sweepers: https://developer.hashicorp.com/terraform/plugin/sdkv2/testing/acceptance-tests/sweepers