[Task]: Beam : Document plugin development guidelines

### What needs to happen?

Migration: https://issues.apache.org/jira/browse/HOP-4479

According to my experience, if one wants to develop a new plugin, he/she comes with lots of unknown codes that they don't have any documentation.In practice, it's very hard to figure out functionality of different parts of source code. For example:

How transforms translated to run on Beam?(e.g, TextFile Input). we need top down approach to figure out what's happening.
How we can develop transform to run on Beam? (e.g, what's handler and how to develop one)
Explanation of Data, Dialog, Meta classes which are common to develop transforms.

### Issue Priority

Priority: 3

### Issue Component

Component: Documentation

Start with the Beam migration HOP-4479 and trace the TextFile Input example to understand how transforms run on Beam. Document the top-down flow, handler development, and the roles of Data, Dialog, and Meta classes; done means a newcomer can use these guidelines to develop a Beam plugin.

Written by the indexing model from the issue text.