docs(user_manual): add CalDAV/CardDAV URL formats to sync guides

[Carsons-Eels]

☑️ Resolves

• Fix CalDAV sync for external clients is not clearly documented #14122

The sync guides for external calendar/contacts clients don't consistently document the actual CalDAV and CardDAV URL formats users need.

This adds the URL formats (both full subscription and specific-calendar variants) to:

• Thunderbird sync page
• macOS sync page
• KDE sync page (previously URLs were only visible in a screenshot, inaccessible to screen readers)
• Calendar app page (new section on connecting external clients via CalDAV, plus a tip about the "Internal link" in the calendar edit menu)

iOS and Windows 10 pages already had this info, so those are untouched.

📝 Note

This is my first PR to this repo, and it is unpolished and I probably missed a few things that I need to do. I saw that there are translations into many languages, and I will iterate on this to see what I need to do to get these changes translated.

I'll keep reading up on how to contribute properly. Let me know if you have suggestions, criticisms, or thoughts about the changes. 🖤

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)