Limit when CI workflows are triggered

* skip test workflows if only documentation is changed

* skip Cirrus CI and AppVeyor workflows if only .github directory
  content is changed

* avoid double testing

.appveyor.yml

@@ -1,3 +1,12 @@
+branches:
+  only: [ main ]
+
+skip_commits:
+  files:
+    - '.github/**/*'
+    - 'doc/**/*'
+    - '**/*.md'
+
 environment:
   matrix:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015

.cirrus.yml

@@ -1,8 +1,10 @@
 task:
+  skip: "changesIncludeOnly('.github/**' , 'doc/**', '**.md')"
+
   name: FreeBSD
   freebsd_instance:
     matrix:
-      image_family: freebsd-13-1
+      image_family: freebsd-13-2
       image_family: freebsd-12-3
   install_script: |
     pkg install -y cmake git

.github/workflows/static_analysis.yml

@@ -1,6 +1,11 @@
 name: Static Analysis
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [ main ]
+    paths-ignore: [ 'doc/**', '**.md' ]
+  pull_request:
+    paths-ignore: [ 'doc/**', '**.md' ]
 
 jobs:
   build:

.github/workflows/test-macos.yml

@@ -1,6 +1,11 @@
 name: macOS
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [ main ]
+    paths-ignore: [ 'doc/**', '**.md' ]
+  pull_request:
+    paths-ignore: [ 'doc/**', '**.md' ]
 
 jobs:
   build:

.github/workflows/test-ubuntu.yml

@@ -1,6 +1,11 @@
 name: Ubuntu
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [ main ]
+    paths-ignore: [ 'doc/**', '**.md' ]
+  pull_request:
+    paths-ignore: [ 'doc/**', '**.md' ]
 
 jobs:
   build:

.github/workflows/test-windows.yml

@@ -1,6 +1,11 @@
 name: Windows
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [ main ]
+    paths-ignore: [ 'doc/**', '**.md' ]
+  pull_request:
+    paths-ignore: [ 'doc/**', '**.md' ]
 
 jobs:
   build: