Update Arch Linux instructions

The current Arch Linux instructions [on the website](https://keybase.io/docs/the_app/install_linux#arch-linux) read:

> ## Arch linux
>
> ```sh
> pacaur -S keybase-bin # or your preferred AUR install method
> run_keybase
> ```
>
> Remember to `run_keybase` after you install or upgrade Keybase.
>
> Note: the `community/keybase` and `community/kbfs` packages also exist, but they don't include the GUI or the `run_keybase` script, and they require more manual setup.

These instructions are a bit outdated:

- The `pacaur` AUR helper is [no longer maintained](https://bbs.archlinux.org/viewtopic.php?pid=1755144#p1755144). There is of course no one replacement, and we can bikeshed all day about which AUR helper to recommend, but `aurman` seems to be a popular one which still uses the same syntax (i. e. `aurman -S keybase-bin` works).

- There is also a `community/keybase-gui` package. I now have the three packages from `community/` installed, instead of `keybase-bin`, and they seem to be working fine. However, that’s on a system where Keybase was already set up previously (I reinstalled Linux but backed up my dotfiles, so `~/.config/keybase` etc. were still there), so I can’t say anything about the setup required.

- (While we’re at it – the Linux in Arch Linux is usually capitalized.)

Open the Arch Linux section of the linked installation page and compare its package instructions with the current Arch community and AUR packages. Update the outdated helper reference, package/setup notes, and capitalization while preserving the documented alternatives; done means the website instructions accurately describe the current installation paths.

Written by the indexing model from the issue text.