[5.1] Article Schemaorg plugin

[brianteeman]

Summary of Changes

Simple PR to add the most basic type of schema - Article

Testing Instructions

Install prebuilt package or discover->install and publish plugin

Check that article is in the list of available schema in a content item
Add some values, save the article and then test the generated json-ld markup on the page at https://search.google.com/test/rich-results

Link to documentations

Can't find any documentation for the schemaorg plugins. If you can point me at it then I can add this new schema

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[sandewt]

Possibly a useful link to support testing: https://magazine.joomla.org/all-issues/september/rich-snippets-in-joomla-5

[fgsw]

I have tested this item ✅ successfully on 2f5f1c5

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[viocassel]

I have tested this item ✅ successfully on 1d4e1b4

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[richard67]

I've restored the results from previous human tests as the commits after that were just version number changes, which I've all reviewed right now.

[richard67]

RTC

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[LadySolveig]

Thank you @brianteeman
And also many thanks to all who have participated here with tests and informations @sandewt @fgsw @viocassel @richard67