docs: add Tromsø KDE Plasma keyboard shortcuts cheat sheet

[hanthor]

Summary

• add a dedicated Plasma 6 keyboard-shortcuts cheat sheet for Tromsø
• cover window management, KRunner, virtual desktops, Activities, application shortcuts, and changing bindings
• link the guide from Tromsø Getting Started and the docs sidebar
• remove the stale Ctrl+F1–F4 shortcut claim from Getting Started

Notes

The guide distinguishes Plasma defaults from configurable or application-specific bindings and links to KDE’s current documentation and UserBase references.

Validation

• git diff --check
• source and sidebar link review
• npm ci / Docusaurus validation attempted but blocked by the workspace filesystem reaching 100% capacity

Closes #255

[hanthor]

Triage note — keeping this one. It is not superseded, unlike most of the other August drafts I closed today.

Verified against current main: docs/tromso/ contains CONTRIBUTING.md, ROADMAP.md, SECURITY.md, SPEC.md, ci-and-iso-pipeline.md, getting-started.md, index.md, kde-python-bindings-investigation.md — no shortcuts page. docs/gurnard/pantheon-shortcuts.md exists for Pantheon, so Plasma is the gap this fills.

The correction to docs/tromso/getting-started.md is worth having on its own: main currently claims Ctrl+F1-F4 for virtual desktops, and this PR removes that row precisely because the new page explains that mapping is a pre-Plasma-6 default and may be unset or reassigned. That is a real accuracy fix, not a reword.

Two things before it can land:

1. Still a draft — needs to be marked ready for review.
2. Small conflict incoming — ci(docs): validate documented Homebrew packages #362 merged today and also touches docs/tromso/getting-started.md (adding a # no-verify: annotation to the brew install kde-themedb line, since kde-themedb 404s on the Homebrew formula and cask APIs). Rebasing should be mechanical; keep both changes.

I did not do the rebase because the PR head is on a fork this session cannot push to. Related issue: #255.

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