docs(desktop): improve installation guide structure and desktop client index

[joshtrichards]

☑️ Resolves

This PR refreshes the desktop client user docs with a clearer structure and more consistent wording.

The main focus is the installation guide, which has been reorganized so that the common user path comes first and advanced Windows deployment details appear later in the chapter. I also cleaned up the desktop client index page to make the section landing page shorter, clearer, and more consistent with the updated installation docs.

What changed

user_manual/desktop/installation.rst:

• split the opening section into clearer subsections to make the page easier to scan
• moved advanced Windows MSI/deployment details to the end of the chapter
• added cross-references so the advanced Windows section remains easy to find
• renamed the setup-focused section to Initial Setup
• updated the Linux installation wording to reflect the current AppImage-based
  official distribution model
• clarified the supported server versions wording
• improved wording, flow, and readability throughout
• cleaned up some reStructuredText formatting and indentation

user_manual/desktop/index.rst:

• simplified the intro text
• removed repetitive wording
• aligned terminology with the rest of the desktop client docs

Why

The previous installation page mixed together:

• general download information
• platform-specific installation notes
• initial setup steps
• advanced Windows deployment details

That made the chapter harder to scan, especially for regular users who just want to install the client and complete the first-time setup.

This revision tries to make the structure more intuitive:

• general information first
• platform-specific install notes next
• initial setup flow after installation
• advanced Windows deployment options last

Notes for reviewers

• I tried to keep the actual content intact where possible and focus mainly on structure, wording, and discoverability. Content changes were to address actual errors or clearly outdated information (i.e. Linux download/installation guidance).
• The Windows deployment section is still linked from earlier in the chapter so admin/enterprise readers can find it quickly.

🖼️ Screenshots

[joshtrichards]

/backport to stable33

[skjnldsv]

Nice refactor