doc: add disclaimers for github repo

.github/CONTRIBUTING.md

+This project is tracked on our hosted gitlab server at:
+https://git.duniter.org/nodes/rust/duniter-v2s
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests and issues must be created on the main gitlab repository.

.github/ISSUE_TEMPLATE.md

+Please note that all issues must be opened at https://git.duniter.org/nodes/rust/duniter-v2s/issues/new?issue
+
+If you open issue here, it will be closed.
+
+Thank you for your understanding.

.github/PULL_REQUEST_TEMPLATE.md

+This project is tracked on our hosted gitlab server at: https://git.duniter.org/nodes/typescript/duniter/gitlab
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests proposed on github will be closed.
+
+Thank you for your understanding.

.github/github_disclaimer.md

+# Initial disclaimer
+This project is tracked on our hosted gitlab server at:
+https://git.duniter.org/nodes/rust/duniter-v2s
+
+The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
+
+All contributions should be performed on the main gitlab repository.
+
+Pull requests and issues must be created on the main gitlab repository.
+
+# Original README.md
+