Added Threema Brand

[LukasQ]

Proposed change

Add the Threema Brand for my new core integration. Official source: https://threema.com/de/press

Type of change

• Add a new logo or icon for a new core integration
• Add a missing icon or logo for an existing core integration
• Replace an existing icon or logo with a higher quality version
• Replace an existing icon or logo after a branding change
• Removing an icon or logo

Additional information

• Link to code base pull request: New: Threema Integration core#165993
• Link to integration documentation on our website: Threema Integration Docs home-assistant.io#44160

Yes, they are square ;)

Checklist

• The added/replaced image(s) are PNG
• Icon image size is 256x256px (icon.png)
• hDPI icon image size is 512x512px for (icon@2x.png)
• Logo image size has min 128px, but max 256px, on the shortest side (logo.png)
• hDPI logo image size has min 256px, but max 512px, on the shortest side (logo@2x.png)

[LukasQ]

Updated reference to docs PR: home-assistant/home-assistant.io#44160

[LukasQ]

Created a new core PR as FUBAR-ed my local git config.