feat: add predefined template vairable ds and function ds_add

[jczhong84]

Adding below predefined variable and function

• variable {{ ds }}: same as yesterday
• function {{ ds_add(ds, days) }}: add/sub days from ds

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rendered as

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rendered as

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rendered as

[czgu]

thanks JC! I think the format for ds_add is macros.ds_add.

Also great job for updating the docs, could you have a separate section on Airflow variables and include this link https://airflow.apache.org/docs/apache-airflow/stable/templates-ref.html