transform work, some documentation

[fivetran-jamie]

Are you a current Fivetran customer?

fivetran made PR

What change(s) does this PR introduce?

• light and INCOMPLETE docs work. This is definitely in progress, and the docs work here is just stuff i've been able to do in my free time
• fixes up the new fields in customer (re https://github.com/fivetran/dbt_shopify_source/issues/46)
• identifier vars for new tables
• added a new table (abandoned_checkout_shipping_line)
• consistent casing of things we join on (ie email and discount code)

Did you update the CHANGELOG?

• [ ] Yes

a little but this will be completed in a later PR

Is this PR in response to a previously created Bug or Feature Request

• [x] Yes, Issue/Feature https://github.com/fivetran/dbt_shopify_source/issues/46
• [ ] No

How did you test the PR changes?

• [ ] Buildkite
• [ ] Local (please provide additional testing details below)

Select which warehouse(s) were used to test the PR

• [x] BigQuery
• [x] Redshift
• [x] Snowflake
• [x] Postgres
• [x] Databricks
• [ ] Other (provide details below)

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:dancer: **Feedback** We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your [feedback](https://www.surveymonkey.com/r/DQ7K7WW) on our existing dbt packages or what you'd like to see next.

[fivetran-jamie]

this doesn't need to be merged ASAP, just want eyes on it/aware of these changes as we work more on the transform package