Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
Changes:
* Moves sample data to reusable rest test
* Combines EQL index, requirements, and run a search pages
* Combines EQL syntax and limitations pages
* Adds related redirects
Changes:
* Swaps the `dev` admonitions for `experimental` admonitions
* Removes `ifdef` statements preventing the docs from appearing in
released branches
