Reinforce visibility of official Laravel courses in introduction

[ivanmercedes]

This pull request adds a brief mention of the official Laravel courses
in the introduction section.

Although these courses are already listed under the Resources menu, this addition aims to make them more visible to new users reading the documentation for the first time. Many beginners may assume that Laracasts is the only learning path mentioned, so referencing the official free courses directly here helps clarify that Laravel also provides its own guided learning experience.

This small change improves the onboarding experience without altering the structure of the documentation.