add tutorial for customized view

[zhangmeng916]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Fixes #1144

Description

• Here are some details about my PR, including screenshots of any UI changes:

Add tutorial for how to use customized view

Tests

• The following tests are written for this issue:

mvn site:site
mvn site:stage
passed, and the generated website is checked

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[zhangmeng916]

This PR is ready to be merged, approved by @dasahcc
Final commit message:
Add tutorial for customized view.