docs: add platform-specific install guides, release process, and docs index

[q-soriarty]

Description

Essential documentation modeled after gh CLI docs/ structure, needed before v1.0.0.

Tasks

• docs/README.md — docs folder index with links to all docs
• docs/install_linux.md — Linux install guide (deb, rpm, Homebrew, binary, source)
• docs/install_macos.md — macOS install guide (Homebrew, binary, source)
• docs/install_windows.md — Windows install guide (binary, source, future winget)
• docs/install_source.md — Build from source + cross-compilation
• docs/releasing.md — Release process guide (tag, GoReleaser, Homebrew, changelog)

Reference

Based on gh CLI docs/: install_linux.md, install_macos.md, install_windows.md, install_source.md, releasing.md, README.md

[q-soriarty]

Superseded by #132 (Jekyll documentation migration). Will be closed when #132 is merged.

[github-actions]

Closed by #136 (merged into develop).