Let's review what happens to links in Markdown files.
Markdown such as [contact someone](./contact.md) gets turned into an HTML link to
a page in your repository. The part
of the link in square brackets, contact someone in this case, becomes the anchor text (the
visible part of a link, and the part that gets indexed by search engines). The part in
parentheses, ./contact.md in this case, becomes a reference to an HTML file by the same
name, which would be contact.html in the current directory.
The format for linking to other websites is similar to Markdown but you just use the website's actual URL in parentheses. For example:
Try [Google](https://www.google.com/) for more info.