Add Case conversion functions to the article (#26922)

* Add Case conversion functions to the article

These functions tell the programmers about converting their string data type value either into upper case letters or lower case letters.

* Fix formatting and phrasing

guide/english/java/strings/index.md

@@ -220,5 +220,42 @@ The result will be:
 Free
 ```
 
+
+#### String Case Conversion
+
+1. `.toLowerCase()` - This method will convert the string into lower case characters.
+   Example:
+   ```java
+   String text1 = "Welcome99";
+   String text2 = "WELCOME";
+   String text3 = "well";
+   System.out.println(text1.toLowerCase());
+   System.out.println(text2.toLowerCase());
+   System.out.println(text3.toLowerCase());
+   ```
+   The output will be:-
+   ```
+   welcome99
+   welcome
+   well
+   ```
+2. `.toUpperCase()` - This method will convert the string into upper case characters.
+   Example:
+
+   ```java
+   String text1 = "Welcome99";
+   String text2 = "WELCOME";
+   String text3 = "well";
+   System.out.println(text1.toUpperCase());
+   System.out.println(text2.toUpperCase());
+   System.out.println(text3.toUpperCase());
+   ```
+   The output will be:-
+   ```
+   WELCOME99
+   WELCOME
+   WELL
+   ```
+
 **More Information:**
 - [String Documentation](https://docs.oracle.com/javase/7/docs/api/java/lang/String.html)