Manuals: Improve design and use TreeExpander

[AkshayWarrier]

This PR aims to make improvements to the Documentation Viewer.

• Port the old UI to the new libadwaita 1.4 widgets. The Documentation Viewer now uses a NavigationSplitView for the sidebar.
• Sidebar now shows not only namespaces but also sections and subsections for each namespace using collapsible TreeExpanders

[AkshayWarrier]

This is not ready to be reviewed yet, I might have an idea why it's not working. I want to try something first :)

[AkshayWarrier]

Some things to consider

• There is no section for Function Macros like DevHelp and they are added with the Functions section because both files are named as func.xx.html and I have no way to differ between them
• We could try synchronizing the item selected in the sidebar and the doc shown by the webview? Sometimes the user may click on links on the doc that redirect them to some other page. (Will have to think how to do this though)

[AkshayWarrier]

For some reason, I'm unable to change the default selected item in the sidebar when the doc viewer is first opened.

Scrolling in the web view is broken and also switching between sections in the documentation (when the sections are on the same page) potentially because of https://bugs.webkit.org/show_bug.cgi?id=260073

Edit: This issue is not persisting anymore after updating org.gnome.Platform//master (i think)

[sonnyp]

Looks great!

I pushed minor changes, make sure to git pull.

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I think we should remove empty sections like Bitfields and Error Domains in this example.

If you prefer, you can create a follow up ticket.

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When navigating back, it should select the relevant sidebar item.

This should probably be a follow up though.