Docs: review and update glab packages and glab container-registry documentation
## Summary
Review and update the CLI documentation for the new `glab packages` and `glab container-registry` command groups once all related MRs are merged.
## Background
The `glab packages` and `glab container-registry` command groups adds generic package registry support (`list`, `upload`, `download`, `delete`). This work item tracks that documentation review.
## Related issues and merge requests
`glab packages` only:
- Implementation work item: #7421 (Add Generic Package Repository access)
- gitlab-org/cli!3393 — packages command group + `list` (merged)
- gitlab-org/cli!3394 — `upload`
- gitlab-org/cli!3395 — `download`
- gitlab-org/cli!3396 — `delete`
- gitlab-org/cli!2069 — securefile `get` (the command the packages group was modeled on; merged)
## Proposed tasks
- [x] Review the generated command docs in `docs/source/` for the packages and container-registry commands
- [x] Verify they follow the [glab documentation style guide](https://docs.gitlab.com/development/documentation/cli_styleguide/)
- [x] Run `make gen-docs` and commit any updated files
- [x] Fix any inaccuracies in command/flag descriptions or examples
issue