Beef up MEVD docs #51443
Description
Describe the issue or suggestion
Port vector DB docs from What are Semantic Kernel Vector Stores? (Preview) | Microsoft Learn to this repo, but review and make improvements as well. MEVD has evolved since the docs were written.
We already have https://learn.microsoft.com/en-us/dotnet/ai/conceptual/vector-databases but it's woefully short (not a single code sample etc.). We might add a new top-level section (at the level of tokenization, evaluation...) to house the new content.