Rename local install docs to 'Local (Self-Hosted)'

[lin-bot23]

Summary

Renames the local installation section and doc titles to clearly indicate they are the local self-hosted alternative to Comfy Cloud.

Changes

Navigation (docs.json):

• Install locally → Local (Self-Hosted) (en)
• 本地安装 → 本地 (自托管) (zh)
• ローカルインストール → ローカル (セルフホステッド) (ja)

Document titles (en):

File	Title
system_requirements.mdx	System Requirements — Local Self-Hosted
desktop/windows.mdx	Windows Desktop — Local Self-Hosted
desktop/macos.mdx	MacOS Desktop — Local Self-Hosted
desktop/linux.mdx	Linux Desktop — Local Self-Hosted
comfyui_portable_windows.mdx	ComfyUI(portable) Windows — Local Self-Hosted
manual_install.mdx	Manual Installation — Local Self-Hosted
update_comfyui.mdx	How to Update ComfyUI — Local Self-Hosted

Sidebar titles also updated to append (Local) for consistency.

Motivation

Make it clear to users that these are the local download alternative to Comfy Cloud. The Self-Hosted label clearly contrasts with Cloud and helps users understand their options.