Add a social icon for Wikipedia

What does this PR change? What problem does it solve?

Usage:

socialIcons: - name: wikipedia | wiki url: "https://en.wikipedia.org"

Was the change discussed in an issue or in the Discussions before?

Closes #1530 (FYI: @tobiabocchi)

PR Checklist

[ ] This change adds/updates translations and I have used the template present here.
[x] I have enabled maintainer edits for this PR.
[x] I have verified that the code works as described/as intended.
[x] This change adds a Social Icon which has a permissive license to use it.
[x] This change does not include any CDN resources/links.
[x] 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.