[github] update issues and PR templates

[lgrd]

Need

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

Tasks

• 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
• Update the documentation
  • changelog.md
  • documentation directory
• If possible, rebase your branch from develop

Code

I only edited the template files.