docs: add some nuxt concepts (#141)

Author: ubugeeei

content/2.concepts/6.server-side-rendering/index.md

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+---
+ogImage: true
+---
+
+# Server Side Rendering
+
+<!-- TODO:
+  - What is Server Side Rendering?
+    - pros and cons
+  - How to implement Server Side Rendering in a Nuxt.js app
+  - Actual example
+-->

content/2.concepts/7.state-manegement/index.md

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+---
+ogImage: true
+---
+
+# State Management
+
+<!-- TODO:
+  - What is State Management?
+    - Problems on SSR
+  - How to implement State Management in a Nuxt.js app (useState)
+-->

content/2.concepts/8.data-fetching/index.md

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+---
+ogImage: true
+---
+
+# Data Fetching
+
+<!-- TODO:
+  - What is Data Fetching?
+  - useFetch, $fetch, useAsyncData
+  - SSR (default)
+  - CSR
+  - SSG
+  - ISR (?)
+-->

content/2.concepts/9.error-handling/index.md

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+---
+ogImage: true
+---
+
+# Error Handling
+
+<!-- TODO:
+  - Error Handling in Nuxt.js
+  - error.vue
+  - useError, createError
+  - NuxtErrorBoundary
+-->