Broken links in introduction document

[t-persson]

Description

Some links are broken in the introduction document.
https://github.com/eiffel-community/eiffel/blob/master/introduction/introduction.md

Motivation

Documentation should not have broken links.

Exemplification

Broken links mean documentation is not complete

Benefits

Correct documentation.

Possible Drawbacks

Linking to an external page (sepia) could be "dangerous" as there is no control of it in this repository. But it's better than broken links, which we have now.

[e-backmark-ericsson]

Just as a reference here, the broken links refer to content removed in this PR: #193