The hooks pages sat flat in the learn nav and only documented the LLM
and tool call contexts, with examples built on the legacy decorators.
Groups them under a collapsible "Execution Hooks" section, adds
`step-hooks` and `execution-boundary-hooks` pages covering every hook
context from source, reworks the LLM and tool pages to lead with `@on`
while keeping the decorators, and links the orphaned
`before-and-after-kickoff-hooks` page into the group.
b8cb5dcc69