Include stable package in docs, mention UM, and link to sources for the Fedora package

[lleyton]

Hello, I'm one of the maintainers of the Zed package on Terra. I made the following changes:

• Mention the Terra stable package, instead of only preview and nightly.
• Link to sources for Terra packages instead of pkgs.org.
• Mention Ultramarine in addition to Fedora (one of Terra's targets).

Release Notes:

• N/A

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[lleyton]

@cla-bot check

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