docs(desktop): update account management instructions

[Sudaisib]

☑️ Resolves

• Fixes 15589

📝 Description

This pull request updates the Nextcloud Desktop Client user documentation to reflect the current account management interface and workflow.

The previous documentation referenced the outdated Account > Add New workflow and described configured accounts as tabs within the settings dialog. These instructions no longer accurately reflect the current Desktop Client interface and may make it difficult for users to add, switch, configure, or remove accounts.

This update:

• Updates the Systray Icon section to reflect the current client menu and synchronization options.
• Updates the account settings section to describe the current settings interface.
• Replaces the outdated Account > Add New instructions with the current Add account workflow.
• Documents how users can switch between configured accounts.
• Documents how users can access Settings for a selected account.
• Documents how users can Remove account.
• Improves the documentation terminology to better match the current Desktop Client interface.

These changes are intended to make the desktop user manual clearer and more accurate for users working with recent versions of the Nextcloud Desktop Client.

🖼️ Screenshots

The documentation continues to reference the relevant Desktop Client screenshots. Screenshots should be updated where the current interface differs from the existing images.

✅ Checklist

• I have built the documentation locally and reviewed the output
• Screenshots are included for visual changes
• I have not moved or renamed pages (or added a redirect if I did)
• I have run codespell or similar and addressed any spelling issues