docs: 배경 구매,변경,조회 api명세를 추가한다

docs/api/render/chage_my_background.md

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+## Change my background
+
+배경을 변경합니다
+
+## Request
+### HTTP METHOD : `PUT`
+
+### url : `https://render.gitanimals.org/users/backgrounds`
+
+### PathVariable
+- {username}: 사용자의 이름
+
+### RequestHeader
+- Authorization: `{token}`
+
+## Response
+
+```json
+{
+  "type": "SNOWY"
+}
+```

docs/api/render/get_my_backgrounds.md

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+## Get my background
+
+내가 갖고있는 모든 배경을 조회합니다.
+
+## Request
+### HTTP METHOD : `GET`
+
+### url : `https://render.gitanimals.org/users/{username}/backgrounds`
+
+### PathVariable
+- {username}: 사용자의 이름
+
+## Response
+
+```json
+{
+  "id": "1",
+  "name": "devxb",
+  "backgrounds": [
+    {
+      "type": "SNOWY"
+    },
+    {
+      "type": "WHITE"
+    }
+    ...
+  ]
+}
+```

docs/api/shop/buy_background.md

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+## Buy background
+
+배경화면을 구매합니다.
+
+## Request
+
+### HTTP METHOD : `POST`
+
+### url : `https://api.gitanimals.org/shops/backgrounds`
+
+### RequestHeader
+
+- Authorization: `{token}`
+
+### Response Body
+
+200 OK
+
+```json
+{
+  "type": "SNOWY"
+}
+```

docs/api/shop/get_background.md

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+## Get background
+
+모든 구매가능한 배경을 조회합니다.
+
+## Request
+
+### HTTP METHOD : `GET`
+
+### url : `https://api.gitanimals.org/shops/backgrounds`
+
+### Response Body
+
+200 OK
+
+```json
+{
+  "backgrounds": [
+    {
+      "type": "SHOP",
+      "price": "100000"
+    },
+    {
+      "type": "SNOWY",
+      "price": "100000"
+    },
+    ...
+  ]
+}
+```