docs/provider: Add Environment Variable dictionary to Maintaining Guide (#13518)
docs/provider: Add Environment Variable dictionary to Maintaining Guide (#13518)

* docs/provider: Add Environment Variable dictionary to Maintaining Guide

Initial documentation of all environment variables (beyond standard AWS Go SDK variables) used in the provider acceptance testing. Given the varied implementations and notable extraneous ones, these may warrant future refactoring.

* docs/provider: Add os.LookupEnv() environment variables used in testing and missing `EC2_SECURITY_GROUP_RULES_PER_GROUP_LIMIT`
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws