docs: add 'Set up your editor' to the installation guide

[aurorascharff]

Summary

Adds a "Set up your editor" section to the App Router installation guide. The App Router names files by convention (page.tsx, layout.tsx, route.ts), so an editor quickly fills with same-named tabs. This section shows how to tell them apart with VS Code / Cursor custom editor labels, labeled two folders deep so dynamic routes like blog/[id]/page.tsx do not all collapse to the same [id] label. It includes:

• A copy-paste .vscode/settings.json config (workbench.editor.customLabels.patterns) for setting it up by hand.
• A copyable prompt that points a coding agent back at this section to apply the config, including adapting it to other editors.
• A good-to-know that JetBrains IDEs (WebStorm, IntelliJ) do this automatically.

Verification

• Previewed locally against the docs site: the section renders (HTTP 200), with the config block, the prompt card, and the good-to-know all rendering correctly.

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Pull request overview

Adds an App Router–only “Set up your editor” section to the installation guide to help developers distinguish same-named App Router files (e.g. page.tsx, layout.tsx) via editor tab labeling.

Changes:

• Introduces a new “Set up your editor” section under <AppOnly> in the App Router installation guide.
• Provides a copy/paste VS Code (and Cursor) workbench.editor.customLabels.patterns configuration for two-level-deep folder labeling.
• Adds a copyable “prompt” block for delegating the setup to a coding agent, plus a JetBrains “Good to know” note.

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[icyJoseph]

Looks good.

Most of the review here was about going through the first 3 pages of getting started, thinking how can we make it all better, but for now, let's take this in.