Add informational message about pagination thresholds to the Environments Dashboard page

What does this MR do?

Adds an information message:

This dashboard displays a maximum of 7 projects and 3 environments per project. Read more

to the top of the Environments Dashboard page as described in #39533 (closed).

Screenshots

"Read more" link points to https://docs.gitlab.com/ee/ci/environments/environments_dashboard.html

Closes #39533 (closed)