Port docs off `run_in_actor` + `Portal.wait_for_result`

The 8-page docs sweep of the #477 removal, ahead of the API's
excision,

- `start/quickstart.rst`: the first-actor-tree walkthrough now
  narrates the (migrated) `to_actor.run()` example — no portal
  in hand until the daemon section introduces `start_actor()`.
- `guide/spawning.rst`: the one-shot section becomes
  `to_actor.run()` (blocking call, placement opts, "built on the
  primitives" note); lifetime/teardown rules update — one-shots
  never make it to nursery exit since each is reaped inside its
  own call.
- `guide/rpc.rst`: the `wait_for_result()` section (an API that
  dies with the reap cluster, incl. the `NoResult` sentinel)
  becomes a `to_actor.run()` one-shot section.
- `api/core.rst`: drop `run_in_actor`/`wait_for_result` from the
  autodoc member lists, drop the `Portal.result()` deprecation
  note, add a "One-shot task actors" `tractor.to_actor.run`
  autodoc section.
- `guide/{asyncio,context,cancellation,parallelism}.rst`:
  mention swaps to the successor API.

Gate: `make -C docs html` builds clean; `to_actor.run` autodoc
renders in `api/core.html`.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

docs/api/core.rst

@@ -62,8 +62,8 @@ One-shot task actors
    ``portal=`` it owns only the linked task and leaves the existing
    actor's lifetime to the portal owner; that actor must expose both
    the target module and ``tractor.to_actor.MODULE``. It supersedes
-   the legacy, non-blocking ``ActorNursery.run_in_actor()`` retained
+   the removed (legacy, non-blocking)
-   only for compatibility until its removal in PR #484.
+   ``ActorNursery.run_in_actor()``.
 
 .. deprecated:: 0.1.0a6