[Documentation] Quickstart Web API: Fix Code Typos and API Misuse

[jmprieur]

The Quickstart: Web API documentation (docs/getting-started/quickstart-webapi.md) contains code errors: (1) It uses JwtBearertDefaults.AuthenticationScheme (should be JwtBearerDefaults). (2) The code uses .AddMicrosoftIdentityWebApp(...) instead of .AddMicrosoftIdentityWebApi(...) in Option 2. Both are copy-paste hazards for new adopters.

Required Fixes:

• Correct the typo in the authentication scheme identifier
• Replace AddMicrosoftIdentityWebApp with AddMicrosoftIdentityWebApi where appropriate
• Validate all code blocks for similar/copy-paste inconsistencies

PRs must target: feature/doc-modernization branch.

Affected file: docs/getting-started/quickstart-webapi.md