docs: make KDE custom desktop setup safe and explicit

[4Luke4]

Summary

This documentation-only change removes ambiguity from the custom desktop environment instructions without claiming unsupported runtime behavior.

• Splits the KDE X11 and native Wayland alternatives into separate, valid TOML templates.
• Explains that try_* settings are one-shot and must be renamed after a successful test.
• Documents how to create the optional config file on a fresh installation.
• Corrects the malformed-config description to match the current implementation.
• Clarifies that dependency checks and installation run as root, while the desktop session launches as the configured user.
• Adds a practical KDE troubleshooting table for black/purple screens, fallback behavior, and non-root sessions.
• Marks the historical KDE blog post as archival guidance and points readers to the maintained documentation.
• Records the maintainer-stated project scope for mobile shells rather than publishing unverified Phosh or Plasma Mobile commands.

Issue coverage

• Using other Desktop Environments #150, Can't install KDE #162, and KDE  #289: correct and expand KDE configuration and non-root-user guidance.
• Mobile friendly desktop environments #163 and Feature suggest: Package localdesktop w/ a mobile DE instead of the LXQt #186: clarify the current project scope and avoid presenting unverified mobile-shell presets as supported functionality.

This PR intentionally does not claim that native Plasma Wayland works on every device, and it does not auto-close the linked issues.

Validation

• Parsed every fenced TOML snippet with Python tomllib.
• Checked Markdown code-fence and Docusaurus-admonition balance.
• Verified no TOML example contains duplicate keys.
• Audited behavior against src/core/config.rs, src/android/proot/setup.rs, and src/android/proot/launch.rs.
• Confirmed the branch is based directly on upstream main, is zero commits behind, and changes only the four intended documentation files.

Files changed

• gh-pages/docs/user/3-custom-de.md
• gh-pages/docs/user/4-configurations.md
• gh-pages/docs/user/2-creating-a-non-root-user.md
• gh-pages/blog/2025-07-30-kde-support.md