Add Unix & Linux (Stack Exchange) social icon

[metemaddar]

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

This PR adds the Unix & Linux (unix.stackexchange.com) social icon to the PaperMod theme.
The icon was manually recreated in Inkscape based on the original official image:
https://cdn.sstatic.net/Sites/unix/Img/apple-touch-icon.png

SVG icon:

The new icon allows users to link their profiles on Unix & Linux Stack Exchange.

Was the change discussed in an issue or in the Discussions before?
No prior discussion or issue was found.
This is a simple addition of a new social icon following the existing conventions.

PR Checklist

• This change adds a Social Icon which has a permissive license to use it.
• I have verified that the code works as described/as intended.
• I have enabled maintainer edits for this PR.
• 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 adds/updates translations and I have used the template present here.
• 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

[metemaddar]

Just a quick heads-up: I asked on Unix Stack Exchange Meta about using the (U&L) icon for profile links:
https://unix.meta.stackexchange.com/q/8295/40417

This icon is only for linking to users' Unix Stack Exchange profiles (referral use).
I'll update the PR if they have any guidance or restrictions. Thanks for your patience! 🙏

[metemaddar]

Quick update regarding the Unix Stack Exchange icon used in this PR:

According to Stack Overflow's Trademark Guidance – Flair section:

"DO feel free to use your profile 'Flair' to reference or link to your user account. Use of unaltered logos in the context of displaying the entire content of User Flair is generally okay."

In this PR, the icon is only used to link to users' Unix Stack Exchange profiles (referral context).
This usage is non-commercial and matches the guidance above.

I've also posted on Unix Meta to double-check: https://unix.meta.stackexchange.com/q/8295/40417
I’ll update the PR if any additional guidance comes up.