Support other docsets

[sonnyp]

We currently support gi-docgen docs, we should support other formats.

• Zim - not for dev docs
• DevHelp - deprecated?
• gtk-doc deprecated
• devdocs - there is a gtk3 app for inspiration https://github.com/hardpixel/devdocs-desktop
• docsets - there is a gtk3 app for inspiration https://zealdocs.org/

[AkshayWarrier]

I think the docsets of devdocs make the styling of all the docs uniform and strips the original styling which I don't particularly like.

And as for dashdocs, it does present the docs as it is but the format of the docset and the index it builds from it is not very useful.

For example this is what the index of numpy docs look like

And this what zealdocs creates

It simply dumps everything into these "broad categories" which is not useful to the user because it's impossible to browse, but also it's completely different from the organized sidebar of the native docs.

Imo, if want to provide all these docs offline then the experience of browsing through them should be at least as good as the online native counterparts. And for me personally, I wouldn't want to use an app that either removes all the styling from the docs or it doesn't let me browse the docs the same way as its online version.