docs(java): update JUnit examples with new fixtures

[jfgreffier]

Documentation for JUnit integration aka JUnit fixtures https://github.com/microsoft/playwright-java/issues/1369

Work in progress

• [x] update examples
• [x] list available fixtures in test-runners page
• [x] Add examples and explanations of Options

[jfgreffier]

@uchagani

My understanding is that @TestInstance(TestInstance.Lifecycle.PER_CLASS) is not needed anymore for parallel tests when using @UsePlaywright. Correct me if I'm wrong

[uchagani]

@jfgreffier correct it doesn't matter from a parallel testing perspective. The user can use it if they want but it's more controlling the lifecycle of the test class and not the parallelism.

The user only needs to set up parallelism according to Junit5 docs but playwright currently only supports class-level parallelism and not method-level.

[jfgreffier]

Sounds like a plan.