docs: clarify current KernelSU installation paths

[Zhanfg]

Summary

Update the installation documentation to match the current KernelSU release
and support model.

Since KernelSU v3.0, official generic GKI boot images are no longer published
as the normal installation path. However, the installation documentation still
described downloading a KernelSU-provided generic boot.img from Releases and
presented GKI image mode as a current alternative to LKM mode.

This can send users toward release artifacts that no longer exist.

Changes

• clarify that LKM is the current recommended installation path
• document that official GKI image mode support was discontinued starting with v3.0
• remove instructions implying current releases provide generic boot.img files
• retain GKI image mode information as legacy/self-built reference where appropriate
• align installation.md with the current build documentation and release assets

Testing

• built the documentation site with the repository's VitePress production build
• reviewed the final document for remaining references to KernelSU-provided
  generic boot images

Fixes #3368.