docs: add more ways to setup caveman.

[matthieu-27]

What you want
When i search caveman github on goggle,
i wan't to be able to install caveman directly in my agent
instead of going to claudepluginhub.com, waiting for the cloudflare protect, and go to manual install.

Before/after example

Before:
Every agent has its own path (plugin, extension, rule file, or npx skills add). The full per-agent matrix, all flags, dry-run, and uninstall live in INSTALL.md. A few common ones:

# Claude Code plugin
claude plugin marketplace add JuliusBrussee/caveman && claude plugin install caveman@caveman

# Gemini CLI extension
gemini extensions install https://github.com/JuliusBrussee/caveman

# Cursor / Windsurf / Cline / Codex / 30+ more, via the skills registry
npx skills add JuliusBrussee/caveman -a cursor

# Via the /plugin command
/plugin marketplace add juliusbrussee/caveman

# Install Caveman
/plugin install caveman@caveman

After:
Every agent has its own path (plugin, extension, rule file, or npx skills add). The full per-agent matrix, all flags, dry-run, and uninstall live in INSTALL.md. A few common ones:

# Claude Code plugin
claude plugin marketplace add JuliusBrussee/caveman && claude plugin install caveman@caveman

# Gemini CLI extension
gemini extensions install https://github.com/JuliusBrussee/caveman

# Cursor / Windsurf / Cline / Codex / 30+ more, via the skills registry
npx skills add JuliusBrussee/caveman -a cursor

Already inside a Claude Code session? Same thing with the /plugin command:

/plugin marketplace add JuliusBrussee/caveman
/plugin install caveman@caveman

Why good
Saves times, do not redirect to a third party website that has a cloudflare ddos protection on -_- (and count your visits)
Saves clicks.