update issue templates

[mitzimorris]

Summary:

GitHub has changed issue templates. update https://github.com/stan-dev/docs/blob/master/.github/ISSUE_TEMPLATE.md ?

Current Version:

change this!!!!

[WardBrian]

Do we feel like this repo even needs an issue template per se?

[mitzimorris]

the purpose of the template is to make the issue easy to grok and therefore fix, so providing some guidance on what to say would be helpful.

we should also rethink the set of labels for this repo:

• typo: basic copy-edits - missing/extra determiners, prepositions, misspellings, etc.
• correction/clarification: information is wrong/something could be stated better
• new/missing information: things that should be added
• outdated: docs need to be updated to match latest Stan
• build tools: docs not building properly

note that a lot of folks just submit PRs w/out filing issues - but for people who want to just call out a problem, a template could be useful. the current one is too generic.