Add social icon for Nostr

[aaccioly]

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

Add a social icon for Nostr (https://en.wikipedia.org/wiki/Nostr).
Nostr has no official logo, so I've adapted this version from https://github.com/mbarulli/nostr-logo, which is licensed under CC0.

Preview:

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

[aaccioly]

PS: As my fork is in an organisation repo, apparently there's no option to allow mantainers to push to it directly (see https://github.com/orgs/community/discussions/5634). Nevertheless, I'm happy to grant write access to the repo if necessary.

Also, you can see a working version at https://accioly.dev and I have pushed my modified icons back to the original repo at https://github.com/mbarulli/nostr-logo, so they are now "official".

CC: @adityatelange