Update Pluginlib.rst (backport #5894)

[mergify]

The pluginlib page has some issues.

First, I don't think that the area_node should be part of the library: add_library(${PROJECT_NAME} SHARED src/area_node.cpp)
As proposed here I changed it to be an INTERFACE library.

As a consequence,

ament_export_libraries(
      ${PROJECT_NAME}
    )

does not work any more.

Additionally,

    install(
      DIRECTORY include/
      DESTINATION include
    )

was a duplicate with different destination.

---

This is an automatic backport of pull request #5894 done by Mergify.

[mergify]

Cherry-pick of c87aa57 has failed:

On branch mergify/bp/jazzy/pr-5894
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Unmerged paths:
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	both modified:   source/Tutorials/Beginner-Client-Libraries/Pluginlib.rst

no changes added to commit (use "git add" and/or "git commit -a")

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[github-actions]

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