Use Wasabi S3 and more

Based on !370 (closed).

Changelog

The format is based on Keep a Changelog.

Added

• A section about the Discord CDN in the contribution guidelines.
• A copy of the latest version of Discord's Terms of Service, for potential use with REUSE.
• The PowerShell script I made and used to help me with MR (!370 (closed)).

Changed

• Replace links to files hosted on media.discordapp.net and cdn.discordapp.com to copies of their content on Wasabi.
• Hard link files when building the Zola site instead of copying.

Removed

• Autoplay in videos generated with the video shortcode.

Fixed

• Remove alt arguments with empty values in certain calls to Zola shortcodes.