Improve doc widgets/data post processing + rephrase "Series" section

[OKhalkov]

PR description

Improve doc widgets/data post processing + rephrase "Series" section

Rephrase "section" from:

• In the widget → go to Series section → click the Pencil icon next to the data key.
  to:
• In the widget → go to section with Data keys → click the Pencil icon of the target data key.

Affects: /docs/pe/user-guide/widgets/#data-post-processing-function

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docker run --rm -it --network=host --name=linkchecker ghcr.io/linkchecker/linkchecker --check-extern --no-warnings http://0.0.0.0:4000/