Update PR and issue templates

[ulysses4ever]

• refer to CONTRIBUTING.md
• add templates for things other than bugs (presumably, "enhancement", "documentation")
• ditch mention of v2- commands: they've been long the default
• add simplified PR template (see below)
• refer to master/CONTRIBUTING.md#github-pull-request-conventions in every PR template

Originally posted by @Mikolaj in #6370 (comment)

Simplified PR template

People keep complaining about our PR template in that it has a long set of checkboxes that are not necessarily relevant for small contributions, and those are usually made by newcomers, so they get especially confused. Maybe, we need another template: something like "Documentation" or "User-invisible change" without any checkboxes.

See also:

• stack's templates as a source of inspiration: https://github.com/commercialhaskell/stack/issues/new/choose

[ulysses4ever]

Actually, a reference to CONTRIBUTING.md is already made in the current PR template.

[Mikolaj]

Actually, a reference to CONTRIBUTING.md is already made in the current PR template.

I guess it's only to specific sections, but I agree this is enough. So only the issue templates remain to do.

[ffaf1]

Do we (in general) want a “User questions” template? (edit: Ok, I am seeing what stack does and it correctly points first and foremost to Discourse).

[geekosaur]

I'd say that's either Discourse or possibly the Discussions area. (Might be worth considering that it's very easy to switch something between Discussion and Issue on GH.)

[ulysses4ever]

I don't have a strong feeling about questions. We don't receive many of those anyway. I just felt that it may seem more welcoming to have it. And I'd much rather answer Cabal questions here instead of anywhere else, but I realize that it may be undesirable for some reasons.

[ulysses4ever]

I updated the description with a "simplified PR template". Please, share your thoughts!

[ffaf1]

Where would I choose the simplified template? When I make a PR? Would it be default?

[ulysses4ever]

@ffaf1 when you push the green button, just like with issues, no? I did double-check that GitHub allows several PR templates.