Add ko-fi icon

[stdevel]

This PR adds the ko-fi icon, that is currently missing. After this, it can be used e.g. in the menu.

Pull Request type

Please check the type of change your PR introduces:

• Bug-fix
• Feature (functionality, design, translations, etc.)
• Documentation change
• Project management (tests, CI, GitHub configuration, etc.)
• Other (please describe):

Current state

Currently, the Ko-Fi icon is missing.

Issue Number(s):

This PR adds the missing SVG definition for the icon.

Screenshots, if applicable

Checklist

• Bug-fixes and new features: I have tested locally with the latest release of Hugo extended. This requirement is a standard.
• Bug-fixes, new features, and doc changes: I have updated the relevant documentation as part of this PR. (SVG icons aren't documented currently)
• All PRs: I have signed off (using git commit -s ...), or if not possible due to developer environment constraints, will comment below confirming that I am adhering to the Developer Certificate of Origin.