Update OS-specific repositories in README

[lxp]

• Refactor with collapsable sections
• Check if they are still current
  • Arch Linux
  • FreeBSD
  • NixOS

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-collapsed-sections

For example:

The following installation methods are provided by OS-specific package maintainers and might not provide the latest version.

<details>
<summary>Arch Linux</summary>

#### Arch Linux

...

</details>

[lxp]

@vestingz Your Arch Linux package looks outdated. I think we should update or remove the install instructions before the next release. What do you think?

[vestingz]

@vestingz Your Arch Linux package looks outdated. I think we should update or remove the install instructions before the next release. What do you think?

Hi, do you mean the PKGBUILD from the AUR? It actually will dynamically generate the version number from the latest git commit. See this part in the PKGBUILD: printf "r%s.%s" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"

pkgver=r664.dee5be3

just reflects the version at the time when the PKGBUILD was created. You can ignore it.