docs: describe how to compose Actors

[vdusek]

Stacked on #1155.

Docs for the whole child runs stack, moved out of #1149, #1151, #1153, #1154 and #1155 so those PRs only carry code and tests. Adds sections on named child runs, Actor.child_runs(), aborting child runs with the parent, the concurrency limit and the shared charge budget to the "Interacting with other Actors" guide, each with a code sample.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (fbbcb94) to head (7adac64).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/child-run-budget    #1160      +/-   ##
=========================================================
+ Coverage                  93.44%   93.47%   +0.02%     
=========================================================
  Files                         54       54              
  Lines                       3938     3938              
=========================================================
+ Hits                        3680     3681       +1     
+ Misses                       258      257       -1     

Flag	Coverage Δ
e2e	32.78% <ø> (ø)
integration	51.52% <ø> (ø)
unit	90.42% <ø> (+0.02%)	⬆️

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