Streamlining vector support and PostgreSQL extension documentation

[WoutervdBrink]

I just created a PR that adds information about requiring the pgvector extension to be loaded through Schema::ensureVectorExtensionExists() when using PostgreSQL.

So far, the Schema::ensureVectorExtensionExists() method is currently mentioned in the AI SDK docs (here) and the vector search docs (here. The underlying, but perhaps quite useful Schema::ensureExtensionExists(string $name, ?string $schema = null) method is not documented.

Furthermore, the documentation currently states that only PostgreSQL supports vector search. While this is indeed currently the case (see here), MySQL 9.3 added support for vector types and vector functions, so support could be added there as well.

I think it would be useful to add information about the ensureVectorExtensionExists() and/or ensureExtensionExists methods. I would suggest a separate first-level heading in migrations.md, between 'Indexes' and 'Events', but I'm not too sure about that, which is why I'm opening this discussion rather than immediately submitting a PR.

Furthermore, I would like to know whether it would be considered useful to add vector support for MySQL servers (through the new column type and functions). Such a change would impact the documentation, as it adds a choice between MySQL and PostgreSQL rather than a requirement for PostgreSQL+pgvector.