Updated Arch Linux external references (#22151)

Updates to the Arch Linux subsection within "Unofficial Packages" docs.
External link now points to current package page on archlinux.org, and
the maintainer email address now reflects the official email found in
the Arch PKGBUILD.

Closes #22147

Author: kjkent

site/source/docs/getting_started/downloads.rst

@@ -258,5 +258,5 @@ The following is a partial list of such unofficial emscripten packages:
  - maintainer: @chenrui333
 
 **Arch Linux**
- - package info: https://github.com/archlinux/svntogit-community/tree/packages/emscripten/trunk
- - maintainer: Sven-Hendrik Haase <svenstaro@gmail.com>
+ - package info: https://archlinux.org/packages/extra/x86_64/emscripten
+ - maintainer: Sven-Hendrik Haase <svenstaro@archlinux.org>