docs: add Java version of Best Practices guide

[Riya25maheshwari]

Adds docs/src/best-practices-java.md which was missing for Java while available for Node.js, Python, and .NET.

playwright.dev/java/docs/best-practices currently returns 404.

Translates all code examples to Java using Playwright Java API: JUnit 5 @Test/@beforeeach, AriaRole enums, assertThat assertions, Route.FulfillOptions, and Maven CLI commands.
Removes TypeScript/ESLint sections with no Java equivalent.

[Riya25maheshwari]

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