docs: add @payload-config tsconfig path earlier in installation guide

[SakshixSingh]

fixes #16935

What?

Reorders the "Adding to an existing app" section in docs/getting-started/installation.mdx so users set up the @payload-config TypeScript path alias earlier in the installation flow.

Before: Install packages → Copy Payload files → Add Next.js plugin → Create config + tsconfig path → Start app

After: Install packages → Create Payload config → Add @payload-config to tsconfig.json → Copy Payload files → Add Next.js plugin → Start app

Also adds:

A warning that a missing @payload-config path is a common cause of setup errors
Examples for both ./payload.config.ts and ./src/payload.config.ts

Why?

When adding Payload to an existing Next.js app, users often see many TypeScript/IDE errors across admin and API routes. The copied (payload) files import @payload-config, but the docs previously put the tsconfig setup last — after copying those files and updating next.config. Users followed the docs in order, hit errors, and only later found that adding the path alias fixed everything.

How?

Moved Create a Payload Config from step 4 to step 2
Added a dedicated step for Add @payload-config to your TypeScript config (step 3), before copying Payload files or adding the Next.js plugin
Moved Copy Payload files and Add the Payload Plugin to steps 4 and 5
Added a warning banner and documented both root and src/ config path variants

[SakshixSingh]

Hi! This is a small docs-only change that reorders the installation steps so @payload-config is set up before copying Payload files. Would appreciate a review when someone has a moment. Thanks!