Input System: the Installation documentation still instructs users to install the package from "Unity Registry", which is no longer possible now that the package ships with the Editor

Steps to reproduce:

1. Create a new project with 6000.7.0a6 that bundles Input System 6.7.0
2. In the Project window, expand "Packages" and locate the Input System package
3. Open Documentation~/Installation.md in the package and read the compatibility note at the top
4. Read the "Install the package" section
5. Open "Window > Package Manager", select "Input System", and compare what the window actually offers 
6. The same content can be read as published at https://docs.unity3d.com/Packages/com.unity.inputsystem@1.20/manual/Installation.html, which is byte-identical to the file shipping in 6.7.0.

Actual results: Documentation~/Installation.md is unchanged from 1.20.0 and contradicts the current packaging model in three places.

The "Install the package" section gives a four-step registry install that cannot be performed:

1. In the main menu of the Unity Editor, go to Window > Package Manager to open the Unity Package Manager.
2. Select Unity Registry from the navigation panel.
3. Select the Input System package from the list.
4. Select Install, follow any prompts to enable the backends for the new Input System.

Expected results: The package documentation is revised for the model where Input System ships with the Editor: no registry install instructions

Reproducible with versions: 6.7.0 (6000.7.0a6)

Tested on (OS): MacBook Pro - Tahoe 26.6.2