Use a more subtle color for the survey banner

There's been some feedback that:

The banner on top of our docs has the color of a warning, can we make it more informational so it doesn't look like there is a problem?

The initial idea was to use the same colors we use in the marketing site, taken from https://about.gitlab.com/handbook/markdown-guide/#gitlab-webcast-alert-box.

This MR changes the color to be the same as the help and feedback section.

before	after