Added FAQ page for docs (#80)

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## 💸 TL;DR
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V1 of devvit docs FAQ page. Questions were generated from what was asked
through kappa over the past 6 months.


## 📜 Details
[Design Doc](<!-- insert Google Doc link here if applicable -->)

[Jira](<!-- insert Jira link if applicable -->)

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Author: vdikransamarjian