docs: add pgfence to migration resources

[flvmnt]

Adds pgfence to the existing Migrations resource lists for v7 and v6. It analyzes PostgreSQL Sequelize migration files for lock risks and suggests safer SQL rewrites, complementing migration runners such as Umzug.

I maintain pgfence. The entry links to its public repository and is limited to PostgreSQL migration analysis.

Validation:

• yarn install --immutable, yarn test:format, and yarn test:typings passed.
• Checked the published @flvmnt/pgfence@0.8.1 against Sequelize builder and raw SQL migration fixtures. The raw SQL fixture reported the non-concurrent index write-blocking risk and a CREATE INDEX CONCURRENTLY rewrite.
• git diff --check passed.