chores: update shebang, types, and a outdated link

I'm currently trying to get the project set up to contribute. Just had some chores while reading through the docs.

• In the Contributing Guidelines, the reader is told about the Code of Conduct twice. This merges both sections.
• Fixes various minor typos.
• When a user clones the project, it will be in a podcasts directory, not gnome-podcasts.
• The cargo-vendor repo you linked to is deprecated, as the functionality has been merged upstream.
• The line set -o pipefail is a bashism, so the shebang should be /bin/bash, otherwise some distros will attempt to use dash like in my case which doesn't support it. (ShellCheck)

Let me know if you want me to undo any of these. 👍