Move "Install self-service software" endpoint to public REST API docs (#49618)

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**Related issue:** N/A

# Checklist for submitter

- [ ] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements), JS
inline code is prevented especially for url redirects, and untrusted
data interpolated into shell scripts/commands is validated against shell
metacharacters.

## Summary

Moves the `POST
/api/v1/fleet/device/{token}/software/install/{software_title_id}`
("Install self-service software") endpoint out of the contributor-only
API reference (`docs/Contributing/reference/api-for-contributors.md`)
and into the public REST API docs (`docs/REST API/rest-api.md`), nested
under the existing `## Software` section.

- Added `### Install self-service software` to `docs/REST
API/rest-api.md`, right after `### Uninstall software`, with a TOC entry
and a note that it uses the device's authentication token instead of the
usual Fleet API token.
- Removed the TOC entry and body section for this endpoint from
`docs/Contributing/reference/api-for-contributors.md`. Sibling
device-authenticated self-service endpoints were left in place there
since only this one endpoint was moved.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added an API route allowing Fleet Desktop users to initiate
self-service software installations using a device token and software
title.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Rachael Shaw <r@rachael.wtf>

docs/REST API/rest-api.md

@@ -11653,6 +11653,7 @@ Deletes the session specified by ID. When the user associated with the session n
 - [Create Fleet-maintained app](#create-fleet-maintained-app)
 - [Install software](#install-software)
 - [Uninstall software](#uninstall-software)
+- [Install self-service software by Fleet Desktop token](#install-self-service-software-by-fleet-desktop-token)
 - [Get software install result](#get-software-install-result)
 - [Download software](#download-package)
 - [Delete software](#delete-software)
@@ -12992,6 +12993,27 @@ Uninstalls software from a host.
 
 `Status: 202`
 
+### Install self-service software by Fleet Desktop token
+
+Install self-service software on a macOS, Windows, or Linux host. The software must have a `self_service` flag `true` to be installed.
+
+`POST /api/v1/fleet/device/:token/software/install/:software_title_id`
+
+#### Parameters
+
+| Name  | Type   | In   | Description                        |
+| ----- | ------ | ---- | ---------------------------------- |
+| token | string | path | **Required**. The host's [Fleet Desktop token](https://fleetdm.com/guides/fleet-desktop#secure-fleet-desktop). |
+| software_title_id | string | path | **Required**. The software title's ID. |
+
+#### Example
+
+`POST /api/v1/fleet/device/22aada07-dc73-41f2-8452-c0987543fd29/software/install/123`
+
+##### Default response
+
+`Status: 202`
+
 ### Get software install result
 
 _Available in Fleet Premium._