Add texture pack documentation

[no changelog]

docs/Texture Pack Format.md

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+<!--
+SPDX-FileCopyrightText: 2023 Linnea Gräf <nea@nea.moe>
+
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# Custom SkyBlock Items Texture Pack Format
+
+## Items by internal id (ExtraAttributes)
+
+Find the internal id of the item. This is usually stored in the ExtraAttributes tag (i will soon-ish add a command for
+finding the texture pack id specifically). Once you found it, create an item model in a resource pack like you would for
+a vanilla item model, but at the coordinate `firmskyblock:<internalid>`. So for an aspect of the end, this would be 
+`firmskyblock:models/item/aspect_of_the_end.json` (or `assets/firmskyblock/models/item/aspect_of_the_end.json`). Then,
+just use a normal minecraft item model. See https://github.com/romangraef/BadSkyblockTP/blob/master/assets/firmskyblock/models/item/magma_rod.json
+as an example.
+
+