doc: add GitHub issue templates

[trueNAHO]

About

This is a tracking issue for extending and providing GitHub issue templates for every available issue label.

Closes: #268

Steps

• doc: add GitHub issue templates #516

Unresolved Questions

• What labels should be added or removed?

[danth]

Note: "good first issue" and "help wanted" should not have their own templates, since these are additional labels to be added to issues in other categories. (They only exist since GitHub creates them by default.)

We may also want to adjust the labelling, for example by splitting "feature" into sub-categories:

• New targets
• Adjusting existing targets
• New features in /stylix

[danth]

I would also recommend collecting the templates into a single merge, since it could be confusing for contributors to only see templates for certain issues while we are still busy creating them.

[trueNAHO]

Superseded-by: #516