Docs: Document Transaction Usage within Database Guides

New doc or update?

• New documentation
• Update existing documentation

What

We currently have no documentation around best practices for usage of transactions within GitLab's Database Development Guides.

There are a few specific cases we avoid in our rails codebase, such as avoiding potentially long-lived RPC requests while inside an ActiveRecord transaction or enqueueing ephemeral sidekiq jobs within transactions.

These should be better documented.