chore: add top-level favicon.svg

[EpsilonPrime]

Adds a top-level favicon.svg so tooling (e.g. Conductor) can pick up a Substrait icon.

• Reuses the official Substrait mark from site/docs/img/logo.svg.
• Presented on a rounded square plate filled with #e91e63 — the Material for MkDocs "pink" primary color used by the substrait.io site header — with the mark in black to match the site's header text.

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This change is 

[benbellick]

My understanding is that a favicon is to display up in the corner of the browser here:

When I locally run the website via pixi run mkdocs serve, I don't get that icon. However, if I follow the instructions here then it works locally. Is that the intention? Please let me know if I am misunderstanding.

By the way, what is Conductor?

Thanks!

[EpsilonPrime]

My understanding is that a favicon is to display up in the corner of the browser here:

conductor.build is allows run to run and manage multiple agent workflows simultaneously. Thus the icon needs to be located at the top of the repo (and not the website). It's not a big deal if we don't proceed at this time. It sounds like Conductor isn't getting as much traction out there as I thought and I can always manually assign an icon to each workspace.

[nielspardon]

there is a precedence for such a cosmetic addition in substrait-java: substrait-io/substrait-java#398