docs: add NestJS backend API tutorial

[fungc-io]

Summary

Adds a Get Started → Backend/API Integration tutorial for protecting a NestJS API with the @authgear/nestjs SDK.

• New page get-started/backend-api/nestjs.md — Portal setup (enable Issue JWT as access token), AuthgearModule.forRootAsync with a global guard, @Public() / @CurrentUser(), run & test with curl.
• Registered in SUMMARY.md under Backend/API Integration (next to "Validate JWT in your backend").
• Cross-linked from the generic jwt.md guide.

Placed under Backend/API Integration (not Regular Web App) because the SDK is a resource server — it validates tokens and does not perform the login flow. The tutorial makes that scope explicit and points users to the SPA/mobile guides for obtaining tokens.

Notes

• Links to the example repo authgear/authgear-example-nestjs, which is not created yet — that repo should be published before/with merge.

Test Plan

• Verify the new page renders in GitBook and appears in the nav under Backend/API Integration
• Confirm relative links resolve (jwt.md, ../single-page-app/, ../native-mobile-app/, ../../reference/tokens/jwt-access-token.md)
• Confirm the example-repo link once authgear-example-nestjs exists

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