Fixes: https://github.com/microsoft/playwright/issues/20390
We are doing such a bad job documenting these third party approaches
that they bring more harm than benefit. We should let respective
integration owners own the documentation and link to it. I'll remove it
altogether for now.
- Replace links to `selectors.md` with `locators.md`.
- Remove sections that are covered by Locators docs.
- Restructure the rest of selectors doc, update them in terms of
locators.
Fixes#18992.
