Review and update E2E testing guide in docs

The docs for the end-to-end testing guide need to be reviewed and updated to facilitate engineers outside of Test Platform contributing to the e2e test suite.

• Page object examples need to be updated to reflect the migration to data-testid. - gitlab-org/gitlab!147595 (merged)
• Links need to be updated to the latest org structure. e.g. (https://docs.gitlab.com/ee/development/testing_guide/end_to_end/resources.html#resources-cleanup)
• More data-testid corrections - https://docs.gitlab.com/ee/development/testing_guide/end_to_end/dynamic_element_validation.html, https://docs.gitlab.com/ee/development/testing_guide/end_to_end/style_guide.html, https://docs.gitlab.com/ee/development/testing_guide/end_to_end/feature_flags.html
• Update mobile suite info - https://docs.gitlab.com/ee/development/testing_guide/end_to_end/running_tests_that_require_special_setup.html#guide-to-the-mobile-suite