docs(admin): restructure and update server-side encryption documentation

[joshtrichards]

This PR updates and improves the server-side encryption documentation.

• Adds context about "encryption" to help readers understand Server-Side Encryption (SSE) in relation to other encryption solutions (within Nextcloud and those provided by operating systems, etc.).
• Improves clarity throughout the documentation.
• Reorganizes and streamlines content for better readability.
• Adds additional reference links.
• Fixes outdated comments and notes.
• Expands and clarifies information on key management modes and encryption methods.
• Consolidates encryption troubleshooting entries from the General Troubleshooting chapter, bringing all related troubleshooting content together.

Note: In-depth implementation details remain in the dedicated "Details" chapter, which is not affected by this PR.. Same goes for occ encryption section in the occ command reference chapter.

☑️ Resolves

Follow-up items:

• Review/update Encryption chapter(s) revision(s) #11545
• Review feature: encryption (server-side) tagged issues

🖼️ Screenshots

(works better if you open the screenshot image in a new tab then clicked on again to view it full-size)

[Toorero]

It would be cool if you could mention that occ:fix-encrypted-version will (most likely) lead to data loss on the corrupted files since it will "just" roll-back to the most recent version that has a valid signature discarding more recent versions with signature errors.

[alexanderdd]

Thank you for working on this. Sorry, I don't have time to look at this at the moment.

[joshtrichards]

fix-encrypted-version will (most likely) lead to data loss on the corrupted files since it will "just" roll-back to the most recent version that has a valid signature discarding more recent versions with signature errors.

@Toorero Can you clarify? The "version", in the context of SSE, is the encryption metadata version (not File Versions). The command just tries a bunch of different possible encryption versions. It doesn't touch file content or roll back to any prior file versions.