Add agent onboarding instructions to CLI help and install docs

[begelundmuller]

• Adds a "For Agents" section to the install docs pointing to curl install and rill -h
• Adds agent-specific instructions to rill -h (non-interactive mode only) guiding agents to use rill init
• Adds usage examples to rill init -h

Contributes to https://linear.app/rilldata/issue/PLAT-433/get-claude-to-vibe-code-a-rill-project-without-any-prior-manual-setup

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[begelundmuller]

Note: Will remove the nightly recommendation once I've had a chance to test this (can only test when the updated docs are live).

[royendo]

LGTM!