docs: add the VectorDB step to the v1 to v2 migration guide

[stra1g]

Description

Upgrade an existing v1 deployment, follow this guide exactly, and every vector write fails at runtime with UndefinedColumn (column "content_id" of relation "..." does not exist). Agno v2 added two required columns, content_hash and content_id, to VectorDB tables, but the guide never mentions the migration script that applies them. The "Migrating your Agno DB" section covers only Storage and Memory, and item 4.4 announces the two new columns without saying how to apply them. PgVector.create() does not help: it only creates tables that do not exist, and never alters one that does.

This PR adds:

• A ## Migrating your VectorDB section after "Migrating your Agno DB", mirroring its shape: the script link, what to configure (the database URL and a config listing the schema and the tables to migrate), and notes on idempotency, create() behavior, and the error seen until the migration runs.
• A sentence at item 4.4 routing readers from the column change to the new section.

The migration script (libs/agno/migrations/v1_to_v2/migrate_vectordbs_to_v2.py) has existed since agno-agi/agno#5421 and is currently unreferenced by the guide.

Addresses item 1 of agno-agi/agno#9343.

The remaining two items of that issue (the wrong paths in libs/agno/migrations/README.md, and drift detection in PgVector) are a separate PR against agno-agi/agno.

Type of Change

• Bug fix (errors, broken links, outdated info)
• New content
• Content improvement
• Other: ____

Related Issues/PRs (if applicable)

• Related issue: [Bug] The v1 to v2 VectorDB migration is unreachable: the guide omits it and the in-repo README points to paths that do not exist agno#9343 (item 1)
• Related SDK PR: feat: Migration Manager agno#5421

Checklist

• Content is accurate and up-to-date
• All links tested and working
• Code examples verified (if applicable)
• Spelling and grammar checked
• Screenshots updated (if applicable)