learn: make the links to the author profile clickable

[mcollina]

Each learn page as an author frontmatter. I think it would be really nice to add a backlink to the author github profile.

[mikeesto]

+1. Many of our authors are also missing from https://github.com/nodejs/nodejs.org/blob/main/apps/site/authors.json (for example, you are Matteo)

[AugustinMauroy]

Also mention issue #7093

[ovflowd]

+1. Many of our authors are also missing from main/apps/site/authors.json (for example, you are Matteo)

Feel free to complete that list ;D

[AugustinMauroy]

[!CAUTION] The authors.json file is only valid for the blog section, because the learn section has a valid GitHub username. So don't include the authors of the learn section in authors.json.