Update k6-browser examples with v0.56 `frame.waitForSelector` fix

[heitortsergent]

In the next k6 version release (v0.56), there'll be an update to the frame.waitForSelector API that impacts a lot of other APIs. As a result, we'll be able to simplify a few examples for the k6 browser documentation. For example, an example that uses the waitForPageNavigation method might not need to call that method anymore.

We should test and update the examples for the k6-browser docs so they're concise and work with this new fix.

[heitortsergent]

@ankur22 @tom-miseur I tried to add a description from what I understand of this fix, let me know if there's anything that needs to be added here. 🙇