Refine MR unit tests and contribution guide

Changed the name of the unit tests CI job to accuratly reflect
that both AMD64 and ARM64 tests are executed.

Modified the contribution guide to specify that contributors
should work inside their own fork of the project.

.gitlab-ci.yml

@@ -10,13 +10,12 @@ stages:
   - release
   - distribute
 
-test_amd64:
+unit_test_linux:
   stage: test
   image: docker
   services:
     - docker:dind
   only:
-    - main
     - merge_requests
   before_script:
     - apk update && apk add git