Add more examples/guide around creating reusable test fixtures

[mdcruz]

From https://community.k6.io/t/any-public-examples-of-xk6-browser-with-reusable-fixtures/5874

Might be helpful to create an example on our doc to demonstrate how we can create shared test data or fixtures.

Example:

• How to share test data?
• How to store different locators for shared elements?

[ppcano]

Related to Data Parameterization examples