[excel] (Samples) Add explicit note about opening the code editor to all samples

[AlexJerabek]

We received feedback on the docs that it wasn't apparent where you were supposed to paste the Office Scripts code samples. This PR aims to make that more apparent.

[learn-build-service-prod]

Learn Build status updates of commit be964c0:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/includes/open-code-editor-single-script.md	✅Succeeded	View
docs/includes/open-code-editor.md	✅Succeeded	View
docs/resources/samples/add-image-to-workbook.md	✅Succeeded	View
docs/resources/samples/automate-tasks-on-all-excel-files-in-folder.md	✅Succeeded	View
docs/resources/samples/chart-samples.md	✅Succeeded	View
docs/resources/samples/community-seasons-greetings.md	✅Succeeded	View
docs/resources/samples/conditional-formatting-parameters.md	✅Succeeded	View
docs/resources/samples/conditional-formatting-samples.md	✅Succeeded	View
docs/resources/samples/convert-csv.md	✅Succeeded	View
docs/resources/samples/copy-tables-combine.md	✅Succeeded	View
docs/resources/samples/count-blank-rows.md	✅Succeeded	View
docs/resources/samples/data-validation-samples.md	✅Succeeded	View
docs/resources/samples/email-images-chart-table.md	✅Succeeded	View
docs/resources/samples/excel-calculation.md	✅Succeeded	View
docs/resources/samples/excel-cross-reference.md	✅Succeeded	View
docs/resources/samples/external-fetch-calls.md	✅Succeeded	View
docs/resources/samples/get-table-data.md	✅Succeeded	View
docs/resources/samples/javascript-dates.md	✅Succeeded	View
docs/resources/samples/move-rows-across-tables.md	✅Succeeded	View
docs/resources/samples/range-samples.md	✅Succeeded	View
docs/resources/samples/remove-hyperlinks-from-cells.md	✅Succeeded	View
docs/resources/samples/report-day-to-day-changes.md	✅Succeeded	View
docs/resources/samples/row-and-column-visibility.md	✅Succeeded	View
docs/resources/samples/save-as-pdf-email-as-pdf.md	✅Succeeded	View
docs/resources/samples/table-of-contents.md	✅Succeeded	View

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[learn-build-service-prod]

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