Update guidelines to add a new application setting

What does this MR do and why?

To add a new rate limit or an application setting we should use a JSONB column to store the new setting. It has the following advantages:

1. Prevents the application_settings from getting wider. Postgres has a limit of 1600 columns.
2. We can continue using the rails validations.
3. Store different data types.
4. Adding a new setting to an existing column won't a review from the database team.

Related discussions

• !140633 (comment 1726855037)
• #205669
• #423404 (comment 1711208127)

MR acceptance checklist

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Related to #205669