docs(influxdb3): add API usage examples for backup/restore endpoints

## What

Add HTTP API usage examples to the InfluxDB 3 Enterprise backup/restore docs.

PR #7582 documents the `influxdb3` CLI backup/restore commands (`create backup`,
`create restore`, incremental chains, status/show/delete/cancel) for 3.11's
upgraded storage engine. The CLI subcommands map to
`/api/v3/enterprise/backup[/{name}]` and `/api/v3/enterprise/restore[/{id}]`,
but the docs don't show raw API request examples (curl or client-library) for
readers who script against the API directly instead of the CLI.

## Why

CLI docs are done; API request/response examples were out of scope for #7582
and are being tracked separately.

## Scope

- `content/shared/influxdb3-admin/backup-restore.md` (Enterprise section)
- Cover: create full/incremental backup, check status, list, delete, restore,
cancel — as raw HTTP requests, alongside the existing CLI examples.

Start in content/shared/influxdb3-admin/backup-restore.md, in the Enterprise section, and review the existing CLI examples alongside the mapped /api/v3/enterprise/backup and /api/v3/enterprise/restore endpoints. Add raw HTTP examples for full and incremental backup, status, list, delete, restore, and cancel, with request and response details where documented.

Written by the indexing model from the issue text.