Fix completion source crash in Windows OS

Since source was added in completion, completion doesn't work on Windows OS because of use of Path.getPaths to retreive the file name.

For instance: here the error on Window OS

```
java.nio.file.InvalidPathException: Illegal char <:> at index 4: file:/C:/Users/azerr/.lsp4xml/cache/https/www.w3.org/1999/11/xslt10.xsd
	at java.base/sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182)
	at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153)
	at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77)
	at java.base/sun.nio.fs.WindowsPath.parse(WindowsPath.java:92)
	at java.base/sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:229)
	at java.base/java.nio.file.Path.of(Path.java:147)
	at java.base/java.nio.file.Paths.get(Paths.java:69)
```

This PR fixes this bug on Wondows OS by avoiding using Paths.getPath to retrieve the file name.  I have too cleaned a little the code of generateDocumentation.

org.eclipse.lsp4xml/src/main/java/org/eclipse/lsp4xml/extensions/contentmodel/utils/XMLGenerator.java

@@ -10,8 +10,6 @@
  */
 package org.eclipse.lsp4xml.extensions.contentmodel.utils;
 
-import java.nio.file.Path;
-import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
@@ -141,7 +139,6 @@ public String generate(Collection<CMAttributeDeclaration> attributes, String tag
 
 	private int generate(Collection<CMAttributeDeclaration> attributes, int level, int snippetIndex, XMLBuilder xml,
 			String tagName) {
-		int attributeIndex = 0;
 		List<CMAttributeDeclaration> requiredAttributes = new ArrayList<CMAttributeDeclaration>();
 		for (CMAttributeDeclaration att : attributes) {
 			if (att.isRequired()) {
@@ -162,7 +159,6 @@ private int generate(Collection<CMAttributeDeclaration> attributes, int level, i
 			} else {
 				xml.addSingleAttribute(attributeDeclaration.getName(), value, true);
 			}
-			attributeIndex++;
 		}
 		return snippetIndex;
 	}
@@ -174,7 +170,8 @@ private int generate(Collection<CMAttributeDeclaration> attributes, int level, i
 	 */
 	public static String generateAttributeValue(String defaultValue, Collection<String> enumerationValues,
 			boolean canSupportSnippets, int snippetIndex, boolean withQuote) {
-		return generateAttributeValue(defaultValue, enumerationValues, canSupportSnippets, snippetIndex, withQuote, null);
+		return generateAttributeValue(defaultValue, enumerationValues, canSupportSnippets, snippetIndex, withQuote,
+				null);
 	}
 
 	/**
@@ -187,9 +184,9 @@ public static String generateAttributeValue(String defaultValue, Collection<Stri
 		StringBuilder value = new StringBuilder();
 		String quotation = "\"";
 		if (withQuote) {
-			if(settings != null) {
+			if (settings != null) {
 				quotation = settings.formattingSettings.getQuotationAsString();
-			} 
+			}
 			value.append("=" + quotation);
 		}
 		if (!canSupportSnippets) {
@@ -221,31 +218,39 @@ public static String generateAttributeValue(String defaultValue, Collection<Stri
 	 * Returns a properly formatted documentation string with source.
 	 * 
 	 * If there is no content then null is returned.
+	 * 
 	 * @param documentation
 	 * @param schemaURI
 	 * @return
 	 */
 	public static String generateDocumentation(String documentation, String schemaURI) {
-		StringBuilder sb = new StringBuilder();
-		boolean tempDocHasContent = !StringUtils.isEmpty(documentation);
-		if(tempDocHasContent) {
-			sb.append(documentation);
-		}
-
-		if(schemaURI != null) {
-			if(tempDocHasContent) {
-				String lineSeparator = System.getProperty("line.separator");
-				sb.append(lineSeparator);
-				sb.append(lineSeparator);
+		StringBuilder doc = new StringBuilder(documentation != null ? documentation : "");
+		if (schemaURI != null) {
+			if (doc.length() != 0) {
+				doc.append(System.lineSeparator());
+				doc.append(System.lineSeparator());
 			}
-
-			sb.append("Source: ");
-
-			Path schemaPath = Paths.get(schemaURI);
-			sb.append(schemaPath.getFileName().toString());
+			doc.append("Source: ");
+			doc.append(getFileName(schemaURI));
+		}
+		return doc.length() > 0 ? doc.toString() : null;
+	}
+
+	/**
+	 * Returns the file name from the given schema URI
+	 * 
+	 * @param schemaURI the schema URI
+	 * @return the file name from the given schema URI
+	 */
+	private static String getFileName(String schemaURI) {
+		int index = schemaURI.lastIndexOf('/');
+		if (index == -1) {
+			index = schemaURI.lastIndexOf('\\');
+		}
+		if (index == -1) {
+			return schemaURI;
 		}
-		String finalDocumentation = sb.toString();
-		return !finalDocumentation.isEmpty() ? finalDocumentation : null;
+		return schemaURI.substring(index + 1, schemaURI.length());
 	}
 
 }