Split Widget Plasma QML API in 2 pages, Add C++ Widget Section, Update metadata.desktop to .json

Random

• Fix broken images and alt text for breeze icons on design homepage. https://apps.kde.org/app-icons/org.kde.Help.svg is 404. I chose org.kde.spectacle.svg and org.kde.khelpcenter.svg to replace the duplicate org.kde.Help.svg. (I can easily rebase this into another MR)

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Plasma Docs Random

• Replace the 0xa0 with spaces in the ascii filetree
• Add rowlayout image previously on imgur
• Remove bold and use alert-link for links in the i18n alerts. https://getbootstrap.com/docs/4.6/components/alerts/#link-color There was no contrast between alert color="info" text and normal links in the light theme.
• kickoff ConfigButtons.qml no longer exists
• Fix images in section-right relative to root instead of current path. Eg: iconsizes.png in plasma-qml-api is 404 atm.

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Update metadata.desktop to metadata.json property names while keeping. The Translation page and scripts still require metadata.desktop though, but that's a limitation of xgettext.

• Update #metadatadesktop anchor links to #metadatajson links
• Refactor metadata.desktop section to metadata.json keys. We still keep the old metadata.desktop keys so they'll show up in search. Also add a few filepaths to unlabeled code blocks.
• Add blurb documenting the existing namespaces
• Use the github namespace in the other examples instead of kde calendar

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Moved the .filepath CSS to a snippet imported in all extend/plasma/widget/ pages, and added filename labels to all code blocks. I might squish a few of these commits in a rebase before merging as there's a couple of "missed" commits.

• Move code filepath to snippet. Add filepaths to other code blocks.
• Fix filepath div being wider than code block when viewport >=992px.
• Fix missed filepath in config page (TODO squish)
• Add labels to NoApply config example to demonstrate best practice (TODO squish)
• Add a few missing labels to config examples (TODO squish)
• Refactor metadata.desktop section to metadata.json keys. We still keep the old metadata.desktop keys so they'll show up in search. Also add a few filepaths to unlabeled code blocks.

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Split the huge plasma-qml-api webpage into two pages. Also add another section to begin documenting the C++ QML Modules and compiling a widget with CMake.

• Move the Plasmoid property group and metadata.json to own webpage
• Show Plasmoid group example in the context property before the table
• Start a new C++ widget section with just examples for now.
• Document a few more plasmoid properties including nativeInterface
• Forgot to link systemtray containement example (TODO squish)