Repo housekeeping pre-v1

[jtcohen6]

Bring this repo into line with other Labs-maintained plugins (dbt-redshift, dbt-snowflake, dbt-bigquery).

The lists below are starting points; they're not exhaustive. In all cases, we should follow the examples set in the other plugin repos.

README

• Ensure documentation is up-to-date at docs.getdbt.com (here + here). Then, remove all "documentation" from this readme
• Use latest language + links from other plugin repos

Packaging

• setup.py should use compatibility pin (~=) on dbt-core
• dev_requirements.txt should install latest changes from dbt-core repo via git+
• Remove requirements.txt (I think it's redundant)
• Update release GHA workflow

CI

• Update test/custom/base.py to include full integration test scaffolding, rather than from dbt_adapter_tests import DBTIntegrationTestBase
• Switch main CI from CircleCI to GitHub Actions
• Switch integration CI from CircleCI to GitHub Actions

I began this work in #223, but I got stuck with the integration test switch-over.

[github-actions]

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