Added social icon for Unity

[tool-buddy]

What does this PR change? What problem does it solve?
Added a social icon for the Unity game engine. Useful to reference one's Unity forum account or Unity asset store page.
The icon's source is: https://icons.getbootstrap.com/icons/unity/

Was the change discussed in an issue or in the Discussions before?
No

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud