[Feature] Write migration guide and finalize dbt-adapters documentation

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt-adapter functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Write a migration.md doc and get it checked into this dbt-adapters repo.

Needs to address:

1. Overview of how dbt-adapters is different from dbt-core/dbt/adapters
2. Steps to migrate and example PRs from our managed adapters
3. Likely sharp edges / gotchas
4. Estimated time/effort to migrate

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response