gitlab: add template messages for issues and merge requests

Add template messages to help contributors to create issues and merge
requests that are useful and complete.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1669

.gitlab/issue_templates/Bug.md

@@ -0,0 +1,18 @@
+## Summary
+
+(Summarize the bug encountered concisely)
+
+## Version affected
+
+(Which version of NetworkManager is affected (`nmcli --version`), which distro, and the version of any other relevant software involved)
+
+## Steps to reproduce
+
+## Actual result
+
+## Expected result
+
+## Relevant logs
+
+(Please see the DEBUGGING section of `man NetworkManager` and attach any relevant log)
+

.gitlab/issue_templates/Feature.md

@@ -0,0 +1,5 @@
+## Description of the feature
+
+## Description of the use cases
+
+## References and other resources

.gitlab/merge_request_templates/Default.md

@@ -0,0 +1,19 @@
+## Summary
+
+(Describe what the merge request does)
+
+## Purpose
+
+(Describe why the merge request is useful, which use cases it addresses, which issue it fixes)
+
+## Checklist
+
+Please read
+https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md
+before opening the merge request. In particular, check that:
+
+ - the subject for all commits is concise and explicative
+ - the message for all commits explain the reason for the change
+ - the source is properly formatted
+ - any relevant documentation is up to date
+ - you have added unit tests if applicable