Add SVG logo in docs

**Is your feature request related to a problem? Please describe.**

I feel Zstandard deserves to have a vector logo in SVG, which I’m sure already exists somewhere. As Zstandard promotes compression, perhaps SVG, being a text format, would be an ideal candidate to compression rather than using a rasterised PNG file.

**Describe the solution you'd like**

Simply add the source SVG file of the logo to the repo.

**Describe alternatives you've considered**

I could use an app or AI or my hand to trace the logo if I wanted to, but I think the Zstandard community would benefit from an actual standard SVG logo file.

**Additional context**

N/A