Add PreconditionAttribute docs

docs/_overwrites/Commands/PreconditionAttribute.Overwrites.md

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+---
+uid: Discord.Commands.PreconditionAttribute
+seealso:
+    - linkId: Discord.Commands.ParameterPreconditionAttribute
+remarks: *content
+---
+
+This precondition attribute can be applied on module-level or
+method-level for a command.
+
+[!include[Remarks(PreconditionAttribute.Remarks.Inclusion.md)]]
+
+---
+uid: Discord.Commands.ParameterPreconditionAttribute
+seealso:
+    - linkId: Discord.Commands.PreconditionAttribute
+remarks: *content
+---
+
+This precondition attribute can be applied on parameter-level for a
+command.
+
+[!include[Remarks(PreconditionAttribute.Remarks.Inclusion.md)]]
+
+---
+uid: Discord.Commands.PreconditionAttribute
+example: [*content]
+---
+
+// todo: add example

docs/_overwrites/Commands/PreconditionAttribute.Remarks.Inclusion.md

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+A "precondidtion" in the command system is used to determine if a
+condition is met before entering the command task. Using a
+precondidtion may aid in keeping a well-organized command logic.
+
+The most common use case being whether a user has sufficient
+permission to execute the command.

docs/_overwrites/Common/OverrideTypeReaderAttribute.Overwrites.md

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+---
+uid: Discord.Commands.OverrideTypeReaderAttribute
+remarks: *content
+---
+
+This attribute is used to override a command parameter's type reading
+behaviour. This may be useful when you have multiple custom
+@Discord.Commands.TypeReader and would like to specify one.
+
+---
+uid: Discord.Commands.OverrideTypeReaderAttribute
+examples: [*content]
+---
+
+The following example will override the @Discord.Commands.TypeReader
+of @Discord.IUser to `MyUserTypeReader`.
+
+```cs
+public async Task PrintUserAsync(
+    [OverrideTypeReader(typeof(MyUserTypeReader))] IUser user)
+{
+    //...
+}
+```