Add VectorDB serverless project type icon to available icons #4232

While updating these Scout docs (one of Kibana's testing frameworks), I noticed the VectorDB serverless icon might be missing from the docs-builder repository: https://github.com/elastic/docs-builder/blob/main/docs/syntax/icons.md.

This isn't urgent. On our side, it's a cosmetic change to make the docs look a bit nicer.