Update AUR documentation since the package got moved to Arch Extra (#27)

* Replace AUR documentation with Arch/Pacman

Arch upgraded act from the AUR to its "Extra" repository.

src/SUMMARY.md

@@ -2,7 +2,7 @@
 
 - [Introduction](./introduction.md)
 - [Installation](./installation/index.md)
-  - [AUR](./installation/aur.md)
+  - [Arch](./installation/arch.md)
   - [Chocolatey](./installation/chocolatey.md)
   - [COPR](./installation/copr.md)
   - [GitHub CLI](./installation/gh.md)

src/installation/arch.md

@@ -0,0 +1,7 @@
+# [Arch](https://archlinux.org/packages/extra/x86_64/act/) (Linux)
+
+[![arch-shield](https://img.shields.io/archlinux/v/extra/x86_64/act)](https://archlinux.org/packages/extra/x86_64/act/)
+
+```shell
+pacman -Syu act
+```

src/installation/index.md

@@ -59,7 +59,7 @@ go build -ldflags "-X main.version=$(git describe --tags --dirty --always | sed
 
 `act` is available in below package repositories
 
-- [AUR](./aur.md) (`Linux`)
+- [Arch](./arch.md) (`Linux`)
 - [Homebrew](./homebrew.md) (`Linux`, `macOS`)
 - [Chocolatey](./chocolatey.md) (`Windows`)
 - [COPR](./copr.md) (`Linux`)