Add social icon for Signal

Signal added support for usernames and links. So it seems likely people will want to add an icon to their sites.

The icon in the SVG is a derivative of this:

which is in the public domain and/or has a permissive license.

This is what it looks like:

Screenshot 2024-03-02 at 9 14 03 AM

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

It just adds a SVG icon for Signal.

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

PR Checklist

[ ] This change adds/updates translations and I have used the template present here.
[x] I have enabled maintainer edits for this PR.
[ ] 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.