Added Installation

Author: ken-matsui

pages/docs/_meta.json

@@ -1,5 +1,6 @@
 {
     "index": "Introduction",
     "why-poac": "Why Poac?",
+    "installation": "Installation",
     "usage": "Usage"
 }

pages/docs/installation.mdx

@@ -0,0 +1,30 @@
+## Installation
+
+[![Packaging status](https://repology.org/badge/vertical-allrepos/poac.svg)](https://repology.org/project/poac/versions)
+
+### Homebrew (macOS & Linux)
+
+```sh
+brew install poac
+```
+
+### AUR (Arch Linux)
+
+Poac is available in the AUR thanks to the [poac](https://aur.archlinux.org/packages/poac) package.
+
+It can be installed with the AUR helper of your choice.
+
+```sh
+paru -S poac
+```
+
+### Runtime Requirements
+
+* C++ compiler
+* GNU Make
+* pkg-config
+* mkdir
+* printf
+* cpplint (for `poac lint`)
+* clang-format (for `poac fmt`)
+* clang-tidy (for `poac tidy`)