add reusable styles for membership page and can also use in other sites

Created by: flora8984461

This is also part of https://bugs.eclipse.org/bugs/show_bug.cgi?id=567547 The PR for it is: https://git.eclipse.org/r/c/www.eclipse.org/membership/+/170242

• added "no-gutters-mobile" to remove padding left and padding right on mobile view
• added "fit-content" for button "contact us about membership"

I am not sure should I add to a separate file for membership, since after discussion with @autumnfound , this probably can be used else where.

Signed-off-by: Yi Liu yi.liu@eclipse-foundation.org