Changelog: add links to contributors

Prior to the repo take-over, the changelog was maintained in a PEAR `package.xml` file using a format as expected for that file, which did not allow for links.

Since PEAR support has been dropped in version 3.8.0, the changelog was moved to a Markdown file.

This PR is a first effort to improve the usefulness of the changelog by improving the "giving credit where credit is due" part of the changelog.

A best effort has been made to add links for all contributors named in the changelog.

* Whenever possible, a GitHub profile link has been used.
* For older contributions via PEAR, the PEAR user profile link has been used unless their GitHub username could be determined with a high degree of certainty.

Unfortunately, for some contributions, my code archaeology effort have proven in vain and I have not been able to trace the name of the contributor back to a user profile.

:point_right: If this applies to a contribution **_you_** made, please submit a pull request to update the changelog to include a link to your profile.

Note: this may seem like a lot of effort for a changelog/historical information. However, to me, this is about creating a positive climate open for contributions and that makes it important (to me).