Use get.codex.storage scripts for Join Testnet guide on Linux/macOS

[veaceslavdoina]

We recently created set of scripts to simplify Codex installation and they are located in get-codex repository.

Now, we also have two additional scripts - generate.sh and run.sh, so we have a full parity with scripts in the codex-testnet-starter repository. Also, these scripts has some fixes for Windows (msys2).

It probably make sense to include one-liners to the Join Codex Testnet guide for Linux/macOS

# Linux, macOS, Windows (msys2)
curl -s https://get.codex.storage/install.sh  | bash
curl -s https://get.codex.storage/generate.sh | bash
curl -s https://get.codex.storage/run.sh      | bash

Updated guide - https://docs-update-join-testnet-for.codex-docs-8lf.pages.dev/networks/testnet#sc-guide-web.