Add help text for agent selector on env settings page

What does this MR do and why?

Adds the help text message and link under the Gitlab Agent selector in the project environments page.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Visit a project
2. Click on Operate -> Environments from left side panel
3. Click on New Environment button
4. There should be a help text and link under the Gitlab agent selector box

Related to #454329 (closed)