Add an agent onboarding doc for CLI, skills, signup, and token bootstrap

[davidjonesgtm]

Problem

Right now the docs are spread across multiple places:

• CLI install
• CLI auth
• CLI skills
• account creation
• API token creation

That works for humans, but it does not tell a clear story for agents.

Dependency

This docs work should happen after the product-side onboarding work is done, because the website page needs to describe the real supported flow, not an interim workaround.

Blocked by:

• Add a supported invite signup flow for non-browser clients operately#4386
• Let invited users create an API token without the web UI operately#4387
• After browserless login, GET /api/v2/people/get_me returns 500 operately#4388

What is missing

A single doc that answers:

• Should an agent use the web app? (usually no)
• When do skills matter? (optional guidance layer)
• What is the correct order? (CLI -> optional skills -> signup/join -> token bootstrap -> CLI usage)
• How should invited agents join an org?
• How should agents get an API token without UI steps?

Expected result

A single "Agents: getting started" or equivalent page that explains:

1. install the CLI
2. optionally install skills
3. join or sign up
4. create/bootstrap an API token
5. log in with operately auth login
6. use the CLI from then on

Why this matters

Without this page, the product story stays human-centric even if the backend pieces for agent onboarding exist.