Docs: document --backupdb argument for inv docker.manage (#12617)
Closes #7761
## Summary
- Documents the `--backupdb` argument for `inv docker.manage`
- Describes the backup filename pattern:
`dump_<DD-MM-YYYY>_<HH_MM_SS>__<git-hash>.sql`
- Adds restore instructions with step-by-step commands
Note: #8708 proposes a `restoredb` invoke task to automate restoration.
The manual workflow documented here can coexist with that automation if
it lands.
## Test plan
- [x] Built docs locally with `make PROJECT=dev html` - renders
correctly
- [x] Tested `inv docker.manage --backupdb showmigrations` - backup file
created with documented pattern
Generated with [Claude Code](https://claude.com/claude-code)
Steered and reviewed by @nikblanchet (mostly human)
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Co-authored-by: Claude <noreply@anthropic.com>