Update Arch Linux instructions with pkgconf dependency

[MadLittleMods]

Update Arch Linux instructions with pkgconf dependency. pkg-config (pkgconf) is already noted in the required dependencies and is mentioned on a few other distro instructions already:

website/docs/install/build.mdx

Lines 150 to 156 in 1349209

	Required dependencies:
	- `gtk4`
	- `libadwaita`
	- `gtk4-layer-shell`
	- `pkg-config` or `pkgconf`
	- `gettext`

This is spawning from building Ghostty from source with a fresh Manjaro Linux (Arch-based) install in a QEMU vm:

$ zig build -p $HOME/.local -Doptimize=ReleaseFast
...
error: error: unable to find dynamic system library 'gtk4' using strategy 'mode_first'. searched paths:
...
error: unable to find dynamic system library 'libadwaita-1' using strategy 'mode_first'. searched paths:
...

$ which pkg-config
pkg-config not found

$ pamac install pkgconf

It looks like pkgconf is also available through base-devel

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