Document select-next-occurrence, multi-cursor, delete-line, go-to-line, matching-bracket, and duplicate-line-up shortcuts

[NisargJasani0602]

Summary

Follow-up to #4253 — documents six real, currently-working shortcuts that were never added to KeyboardShortcutModal.jsx:

• Mod-D — select next occurrence
• Mod-Alt-Up/Down — add cursor above/below
• Shift-Mod-K — delete line
• Mod-Alt-G — go to line
• Shift-Mod-\ — jump to matching bracket
• Shift-Alt-Up — duplicate line up (complements the Shift-Ctrl-D
  alias added in Add duplicate-line and scroll-without-moving-cursor shortcuts #4254, which only covers the down direction)

All six already work today via CodeMirror 6 defaults — this PR is documentation-only, no behavior changes.

Deliberately left out Mod-Z/Mod-Y (undo/redo) and Tab/Shift-Tab
(indent) as universally expected defaults not specific to this editor.

Test plan

• npm run lint passes
• npm test -- client/modules/IDE/components/Editor passes
• Manually verified in the running dev server: all six entries
  display correctly with the right platform-specific key combo

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines