.gitignore: standardize and specify entries

We ignore files traced by git by adding entries in .gitignore.

The format of the entries are not standardized and some of entries are
too general so git will also ignore other files with same name, which
is an unwanted behavior.

We should only ignore files which the project strictly requires to be
ignored. Let's standardize and specify entries the entries in
.gitignore to solve the problem. The entries are reordered by their
category. Some unnecessary entries are removed and some general entries
are specified.

Co-authored-by: Paul Tan <pyokagan@gmail.com>

Author: fzdy1914

.gitignore

@@ -1,20 +1,21 @@
-build/
-.gradle/
-.checkstyle
-.settings/
-.idea/
-lib/*
-*.iml
-*.log
-*.log.*
-*.csv
-config.json
+# Gradle build files
+/.gradle/
+/build/
+src/main/resources/docs/
+
+# IDEA files
+/.idea/
+/out/
+/*.iml
+
+# Storage/log files
+/data/
+/config.json
+/preferences.json
+/*.log.*
+
+# Test sandbox files
 src/test/data/sandbox/
-preferences.json
+
+# MacOS custom attributes files created by Finder
 .DS_Store
-./screenshot*.png
-classes/
-/data/
-/bin/
-src/main/resources/docs/
-out/