Introduce subsections in Plugins on Resources page

Goal of the changes: to make it easier to find the plugin you're looking for :)

Notes:

• I added BrushColorSwitch, KanvasBuddy and Shotgun Toolkit Engine for Krita plugins. The first one because it's so simple that I'm sure it's finished, the second because it's really popular and I know it must be already usable, the third one because it was made as a part of the pipeline so it's for sure finished already too ("finished" in all three cases I mean: user-ready).
• I commented out "GDQuest Designer's Tools" because it redirects to the batch exporter, and I believe Krita already has it
• I want to include following plugins too: Krita Photobash Plugin and Toolbox, possibly Newspaper plugin if it has a wider functionality than the new halftone plugin in Krita (or, I could include it only on the stable branch if the halftone filter is not going to be release in 4.4.0).
• I need to check KDE forum as well. There was Timed Exercise plugin, for example.

Questions:

• How do you like those categories? Do they make sense? And which plugin is which, does it make sense too?
• I added "Color selectors" category even though it has one plugin, because I have those vivid memories of lots and lots of plugins for color selectors, not sure why because now I cannot find many :)
• I tried to order categories so that the most useful to most users will appear on top and the most technical on the bottom, and Misc is always at the end as well. I tried to do the same with plugins inside categories.