Add links to git-academy-sandbox and techskills_workshops

We need some introductory context on where to find both the sandbox repo and the Tech skills workshops maerials.

Start by reviewing the repository's introductory training materials and identify where new learners look for resource links. Add context and working links to the git-academy-sandbox repository and the Tech skills workshops materials. Done means both resources are clearly discoverable from the introductory documentation.

Written by the indexing model from the issue text.