feat: add svg icon for linktree

[GiggleSquid]

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

• Add an SVG icon for Linktree (linktr.ee)
• SVG sourced from https://simpleicons.org/?q=linktree

Usage:

socialIcons:
  - name: linktree
    url: "https://linktr.ee/"

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