Adds `rel` parameter to the social icons configuration documentation (#987)

### Prerequisites

Put an `x` into the box(es) that apply:

- [x] This pull request fixes a bug.
- [ ] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

PR #195 added the `rel` parameter to the social icons configuration.
This addition is missing in the documentation

### Issues Resolved

This PR adds the parameter to the list for the social icons
configuration in the documentation (configuration.md)

### Checklist

Put an `x` into the box(es) that apply:

#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already

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Co-authored-by: Luiz F. A. de Prá <luizdepra@users.noreply.github.com>