Update installation docs after Marketplace publication

[redcatbear]

Situation

doc/user_guide.md and doc/demo/plugin-demo.md currently tell users to install the plugin from GitHub release ZIP files and state that Marketplace availability is still future work. After the first Marketplace publication, these instructions will be stale and should prefer the JetBrains Marketplace entry.

Out-of-Scope

• Performing the first Marketplace upload.
• Changing plugin behavior.
• Removing GitHub release artifacts.
• Creating a Marketplace badge before a public Marketplace URL exists.

Acceptance Criteria

1. After the Marketplace entry is public, doc/user_guide.md describes Marketplace installation as the primary installation path.
2. GitHub release ZIP installation remains documented only as a fallback or development path.
3. doc/demo/plugin-demo.md is updated to use the Marketplace installation path for demos.
4. README links and badges are updated only after the real public Marketplace URL is available.
5. All changed documentation links pass the existing broken-link checker.