docs: update Claude Code Desktop install steps to match the current UI (#342) (#525)

The desktop-app note pointed at a Customize > + > "Create plugin and
add marketplace" > "Add from repository" flow (from #98) that no
longer matches the app. The unified Claude Code Desktop app's Code
tab runs real Claude Code sessions, so the same /plugin marketplace
add and /plugin install commands typed above work directly in its
prompt box; the + button also opens a Plugins browser, and
marketplaces are managed under Customize in the sidebar (per
Anthropic's current desktop app docs).

Closes #342