* docs: gpbackup/gprestore S3 plugin

-add gpbackup/gprestore --plugin-config option
-add S3 plugin information
-other minor fixes: add index as object, support table data and metadata for --jobs > 1

PR for 5X_STABLE
Will be ported to MAIN

* docs: review updates for gpbackup/gprestore S3 plugin
-moved S3 links to Notes section
-changed name from S3 plugin to S3 storage plugin
-removed draft comments

* docs: gpbackup s3 plugin change binary plugin name to gpbackup_s3_plugin

* docs: s3 plugin - fix typo