Logging bucket index_configs documentation fixes

[jbuck]

• Fix logging bucket index_configs examples
• Fix log bucket indexing documentation link

This PR fixes documentation for the various logging bucket resources, specifically the index_config examples. It was incorrectly written as a map in the example, but the attributes documentation and tests show that index_config is a block.
This will close two issues:

• index_configs is not supported on google_logging_project_bucket_config even in current version 5.35.0 hashicorp/terraform-provider-google#18536
• Support indexConfigs in google_logging_project_bucket_config hashicorp/terraform-provider-google#13287

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Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 4 files changed, 16 insertions(+), 16 deletions(-))
google-beta provider: Diff ( 4 files changed, 16 insertions(+), 16 deletions(-))

[shuyama1]

Thank you!