Add connections and agent-security entries to the common.ai sidebar

[kaxil]

Three sidebar changes from reviewer feedback on the reorganized common.ai docs.

Securing agent tools sits under Security. The Security entry in Basics is fixed by the provider docs template to the vulnerability-reporting policy, and its prominence makes it read as the place to learn about securing agents. That page cannot change, so the agent-security page now appears directly beneath it in the sidebar, and its opening names both sides of the problem: what the agent can reach (tools, connections, limits, which the provider controls) and what flows in and out (sensitive data in the context when a tool reaches an untrusted source, and the agent's output as untrusted input downstream, which the Dag author controls). The closing checklist gains an output-handling item and a sentence on tools that read untrusted sources. The page leaves the Reliability and operations toctree so it has one parent; the prose link there stays.

Connections is a top-level guide again. The reorg folded the seven connection pages under Models and providers, and readers who go straight to connection setup did not find them. A new connections/index.rst lists each connection type with what it is for and links to its reference page. It links rather than nests, since the pages already belong to the providers toctree.

Operators is now "Operators and decorators". Every @task.* decorator is an operator underneath, and readers looking for the decorators did not open the operators entry. The page heading matches.

No page moves, so no redirects.

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