Add skyvern about CLI command

[Copilot]

No CLI entry point existed to answer "what does this application do?" for new users or agent discovery.

Changes

• skyvern/cli/about.py — New about_app Typer command. Renders a Rich-formatted panel describing Skyvern's purpose, key AI-powered page commands (act, extract, validate, prompt), and links to docs/cloud. Supports --json for programmatic/agent use.
• skyvern/cli/commands/__init__.py — Registers about_app as skyvern about.
• skyvern/cli/__init__.py — Exports about_app from the CLI package.

Usage

# Human-readable
skyvern about

# Machine-readable (follows existing JSON envelope schema)
skyvern about --json

Original prompt

Tell me what this application does?

Created from VS Code.