Render index.* like README.* when it's present in a directory

Resolves #18933

changelogs/unreleased/18933-render-index-as-readme.yml

+---
+title: Make index.* render like README.* when it's present in a repository
+merge_request: 16550
+author: Jakub Jirutka
+type: added

lib/gitlab/file_detector.rb

@@ -8,7 +8,7 @@ module Gitlab
   module FileDetector
     PATTERNS = {
       # Project files
-      readme: %r{\Areadme[^/]*\z}i,
+      readme: %r{\A(readme|index)[^\/]*\z}i,
       changelog: %r{\A(changelog|history|changes|news)[^/]*\z}i,
       license: %r{\A((un)?licen[sc]e|copying)(\.[^/]+)?\z}i,
       contributing: %r{\Acontributing[^/]*\z}i,

spec/lib/gitlab/file_detector_spec.rb

@@ -15,7 +15,11 @@ describe Gitlab::FileDetector do
 
   describe '.type_of' do
     it 'returns the type of a README file' do
-      expect(described_class.type_of('README.md')).to eq(:readme)
+      %w(README readme INDEX index).each do |filename|
+        %w(.md .adoc).each do |extname|
+          expect(described_class.type_of(filename + extname)).to eq(:readme)
+        end
+      end
     end
 
     it 'returns nil for a README file in a directory' do