update: Remove mentions of Send's defunct official instance

[redoomed1]

List of changes proposed in this PR:

(I'll mark as ready for review on or a little before May 24 June 7. Read below for why.)

• Send
  • Remove mentions of Send's official instance as it will be terminated by May 24 June 7
    • Relevant discussion: https://discuss.privacyguides.net/t/remove-the-statement-about-sends-official-instance/27564
  • Link to the project's repository instead of the defunct official instance for the card's primary button
• Document Collaboration page
  • Mention CryptPad's official public instance and add button that links to list of public instances
• Frontends page
  • Move the admonition about Old Reddit from under the Redlib card to under the "Reddit" header since it's not directly related to Redlib
• General Changes
  • Move lines about public instances and more technical information to the description under recommendation cards based on this feedback and to keep card descriptions succinct
  • Spell out abbreviations like "E2EE" for the first instance of the term on the page, then use the abbreviation for the subsequent instances
  • For "Repository" buttons, embed direct links to project's Readme to differentiate them from "Source Code" links

[github-actions[bot]]

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[redoomed1]

https://discuss.privacyguides.net/t/remove-the-statement-about-sends-official-instance/27564/14