[docs]: x-shellscript-per-boot is not properly documented

# Documentation request

Document `text/x-shellscript-per-boot` / `text/x-shellscript-per-instance` / `text/x-shellscript-per-once` in *User-data formats*.

### What’s missing
* A subsection explaining the three “per-\*” script frequencies:
* **per-boot** – runs at every boot
* **per-instance** – runs once per instance (first boot only)
* **per-once** – runs exactly once ever
* Corresponding rows in the Content-Type table (alongside `text/x-shellscript`).
* A minimal multipart example showing all three MIME types.
* Pointers to the handler (`cloudinit/handlers/shell_script_by_frequency.py`), the modules (`cc_scripts_per_boot`, `cc_scripts_per_instance`, `cc_scripts_per_once`), and the on-disk script directories under `/var/lib/cloud/scripts/`.

### Where to update
`doc/rtd/explanation/format.rst` (plus cross-link from `doc/rtd/explanation/modules.rst`).

### Why
The feature is implemented, stable, and surfaced by `cloud-init devel make-mime --list-types`, yet undocumented. Proper docs will prevent user confusion and support load.

### Related
#4197