Update page object docs for data-testid
What does this MR do and why?
Updates docs for the page objects section of the end-to-end testing guide to follow data-testid
kebab and string requirements.
Related to gitlab-org/quality/quality-engineering/team-tasks#2324 (closed) & https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/6491
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Edited by Désirée Chevalier