Use .markdown for page templates (#7126)

Merge pull request 7126

docs/_docs/themes.md

@@ -19,8 +19,8 @@ In the case of Minima, you see only the following files in your Jekyll site dire
 ├── _config.yml
 ├── _posts
 │   └── 2016-12-04-welcome-to-jekyll.markdown
-├── about.md
-└── index.md
+├── about.markdown
+└── index.markdown
 ```
 
 The `Gemfile` and `Gemfile.lock` files are used by Bundler to keep track of the required gems and gem versions you need to build your Jekyll site.

test/test_new_command.rb

@@ -57,7 +57,7 @@ class TestNewCommand < JekyllUnitTest
       capture_output { Jekyll::Commands::New.process(@args) }
 
       new_site_files = dir_contents(@full_path).reject do |f|
-        File.extname(f) == ".markdown"
+        f.end_with?("welcome-to-jekyll.markdown")
       end
 
       assert_same_elements static_template_files, new_site_files