doc: document branching and releasing (#554)

This diff starts to better specify branching and the release process, thus addressing https://github.com/ooni/probe/issues/1506#issuecomment-949623019.

.github/workflows/codeql-analysis.yml

@@ -9,12 +9,14 @@ name: "CodeQL"
 
 on:
   push:
-    branches: [ "master", "release/**" ]
+    branches:
+      - "master"
+      - "release/**"
 
 jobs:
   analyze:
     name: Analyze
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       fail-fast: false

.github/workflows/coverage.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
       - "master"
+      - "release/**"
 jobs:
   test:
     runs-on: ubuntu-20.04

.github/workflows/debianrepo.yml

@@ -3,10 +3,8 @@ name: debianrepo
 on:
   push:
     branches:
-      - "stable"
-      - "release/**"
       - "master"
-      - "debianrepo"
+      - "release/**"
 
 jobs:
   test_386: