Extract snippets from remaining SpreadsheetML tutorials, extract "Packages and Document Parts" section, and fix multiple xmlns errors

[mikeebowen]

closes #295 ("Packages and Document Parts" extracted into snippet)
closes #279 (how-to-retrieve-a-dictionary-of-all-named-ranges-in-a-spreadsheet.md)
closes #278 (how-to-parse-and-read-a-large-spreadsheet.md)
closes #277 (how-to-open-a-spreadsheet-document-from-a-stream.md)
closes #276 (how-to-open-a-spreadsheet-document-for-read-only-access.md)
closes #275 (how-to-merge-two-adjacent-cells-in-a-spreadsheet.md)
closes #274 (how-to-insert-text-into-a-cell-in-a-spreadsheet.md)
closes #273 (how-to-insert-a-new-worksheet-into-a-spreadsheet.md)
closes #272 (how-to-insert-a-chart-into-a-spreadsheet.md)
closes #271 (how-to-get-worksheet-information-from-a-package.md)
closes #270 (how-to-get-a-column-heading-in-a-spreadsheet.md)
closes #269 (how-to-delete-text-from-a-cell-in-a-spreadsheet.md)
closes #268 (how-to-create-a-spreadsheet-document-by-providing-a-file-name.md)
closes #267 (how-to-calculate-the-sum-of-a-range-of-cells-in-a-spreadsheet-document.md)
closes #266 (how-to-add-custom-ui-to-a-spreadsheet-document.md)
closes #297 (Replace https with http in xmlns declarations)

[learn-build-service-prod]

Learn Build status updates of commit d349b89:

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docs/includes/spreadsheet/structure.md	✅Succeeded
docs/presentation/structure-of-a-presentationml-document.md	✅Succeeded
docs/spreadsheet/how-to-add-custom-ui-to-a-spreadsheet-document.md	✅Succeeded
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docs/spreadsheet/how-to-create-a-spreadsheet-document-by-providing-a-file-name.md	✅Succeeded
docs/spreadsheet/how-to-delete-text-from-a-cell-in-a-spreadsheet.md	✅Succeeded
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docs/spreadsheet/how-to-get-worksheet-information-from-a-package.md	✅Succeeded
docs/spreadsheet/how-to-open-a-spreadsheet-document-for-read-only-access.md	✅Succeeded
docs/spreadsheet/how-to-open-a-spreadsheet-document-from-a-stream.md	✅Succeeded
docs/spreadsheet/structure-of-a-spreadsheetml-document.md	✅Succeeded
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docs/spreadsheet/working-with-the-shared-string-table.md	✅Succeeded
samples/spreadsheet/add_custom_ui/cs/Program.cs	✅Succeeded
samples/spreadsheet/add_custom_ui/vb/Program.vb	✅Succeeded
samples/spreadsheet/calculate_the_sum_of_a_range_of_cells/cs/Program.cs	✅Succeeded
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samples/spreadsheet/create_by_providing_a_file_name/cs/Program.cs	✅Succeeded
samples/spreadsheet/create_by_providing_a_file_name/vb/Program.vb	✅Succeeded
samples/spreadsheet/delete_text_from_a_cell/cs/Program.cs	✅Succeeded
samples/spreadsheet/delete_text_from_a_cell/vb/Program.vb	✅Succeeded
samples/spreadsheet/get_a_column_heading/cs/Program.cs	✅Succeeded
samples/spreadsheet/get_a_column_heading/vb/Program.vb	✅Succeeded
samples/spreadsheet/get_worksheetformation_from_a_package/cs/Program.cs	✅Succeeded

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[learn-build-service-prod]

Learn Build status updates of commit 80d201a:

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docs/includes/spreadsheet/structure.md	✅Succeeded
docs/presentation/structure-of-a-presentationml-document.md	✅Succeeded
docs/spreadsheet/how-to-add-custom-ui-to-a-spreadsheet-document.md	✅Succeeded
docs/spreadsheet/how-to-calculate-the-sum-of-a-range-of-cells-in-a-spreadsheet-document.md	✅Succeeded
docs/spreadsheet/how-to-create-a-spreadsheet-document-by-providing-a-file-name.md	✅Succeeded
docs/spreadsheet/how-to-delete-text-from-a-cell-in-a-spreadsheet.md	✅Succeeded
docs/spreadsheet/how-to-get-a-column-heading-in-a-spreadsheet.md	✅Succeeded
docs/spreadsheet/how-to-get-worksheet-information-from-a-package.md	✅Succeeded
docs/spreadsheet/how-to-open-a-spreadsheet-document-for-read-only-access.md	✅Succeeded
docs/spreadsheet/how-to-open-a-spreadsheet-document-from-a-stream.md	✅Succeeded
docs/spreadsheet/structure-of-a-spreadsheetml-document.md	✅Succeeded
docs/spreadsheet/working-with-formulas.md	✅Succeeded
docs/spreadsheet/working-with-tables.md	✅Succeeded
docs/spreadsheet/working-with-the-shared-string-table.md	✅Succeeded
samples/spreadsheet/add_custom_ui/cs/Program.cs	✅Succeeded
samples/spreadsheet/add_custom_ui/vb/Program.vb	✅Succeeded
samples/spreadsheet/calculate_the_sum_of_a_range_of_cells/cs/Program.cs	✅Succeeded
samples/spreadsheet/calculate_the_sum_of_a_range_of_cells/vb/Program.vb	✅Succeeded
samples/spreadsheet/create_by_providing_a_file_name/cs/Program.cs	✅Succeeded
samples/spreadsheet/create_by_providing_a_file_name/vb/Program.vb	✅Succeeded
samples/spreadsheet/delete_text_from_a_cell/cs/Program.cs	✅Succeeded
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samples/spreadsheet/get_a_column_heading/cs/Program.cs	✅Succeeded
samples/spreadsheet/get_a_column_heading/vb/Program.vb	✅Succeeded
samples/spreadsheet/get_worksheetformation_from_a_package/cs/Program.cs	✅Succeeded

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[twsouthwick]

LGTM

[twsouthwick]

@AlfredHellstern did you want to comment at all here? otherwise I'll merge it in

[AlfredHellstern]

LGTM 2