doc: How to migrate from pgvecto.rs to VectorChord

[mertalev]

Related to this:

1. Will there be an expected version of pgvecto.rs when migrating to VectorChord? For instance, will it expect 0.3 or 0.4, or would 0.2 be enough?
2. We were thinking about whether we should support 0.4 given VectorChord will be ready soon, as it's an additional upgrade path that could maybe make the migration more complicated. Do you think this is a reasonable concern, or is it unlikely to be an issue?

[VoVAllen]

@mertalev The migration path might be more robust, but need some extra steps. The suggested way would be:

• DROP all pgvecto.rs indexes
• ALTER all vector columns to real[], so no longer dependency on pgvecto.rs for the whole database
• DROP extension pgvecto.rs
• INSTALL VectorChord+pgvector (Or change to the docker image with it)
• ALTER all vector columns to vector type of pgvector
• CREATE INDEX

And the first two steps should be able done with a SQL function, but need to be done at the old version. This is to eliminate dependency on pgvecto.rs so 0.2 should also work.

I think we're not ready yet and need several tests in the coming week. Target to be ready at Christmas