[github] update issues and PR templates

[lgrd]

Need

There were no templates for PR; and the templates for issues needed a few changes.

Tasks

• [x] Code the feature on a new branch created from develop (see next paragraph)
• [ ] ~Update tests~
  • [ ] utest/itest if modification of a classe
  • [ ] rtest if modification of an API
  • [ ] ctest if modification of a configuration
• [ ] ~Run tests~
  • [ ] Inside the docker provided by the project : rtest, ctest, utest, itest
  • [ ] Manually if necessary
• [x] Update the documentation
  • [x] changelog.md
  • [ ] ~documentation directory~
• [x] If possible, rebase your branch from develop

Code

I only edited the template files.