Update docs about new AvaTax discounts strategies

[lkostrowski]

Requires

• https://linear.app/saleor/issue/SHOPX-1122
• https://linear.app/saleor/issue/SHOPX-760

to be released (3.19, 3.20)

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Name	Status	Preview	Comments	Updated (UTC)
saleor-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 2, 2024 2:59pm