Enhace Getting Started section in Readme

**Is your feature request related to a problem? Please describe.**
The current details in [Getting Started ](https://github.com/intuit/spring-pulsar/blob/master/.github/GETTINGSTARTED.md) is not enough to setup a developement workspace. It just has two steps.
* Cloning the repo
* Executing mvn clean install.

There are more steps and dependecies required tfor a developement workspace setup.
It also does not talk about different lifecycle phases that run on clean install.

In short the current description is not enough for creating a developement workspace.

**Describe the solution you'd like**
Add more details on Getting Started docs, so that it exhaustively details out every dependecy and steps to setup development workspace.

Few details that has to be added are.
* Which version of Java and maven should be present in machine
* How to setup GPG?
* Setup of docker to run pulsar container.
* Executing pulsar container before running maven clean install

The above is not an exhaustive list.

**Additional context**
The list defined above is not an exhaustive list of details that has to be added in Getting Started. We need to do fresh developement setup in a fresh machine and then figure out the steps , dependecies needed for setup.

After that add all the required steps and dependecies details in Getting Started.

Start with .github/GETTINGSTARTED.md and reproduce the development setup on a fresh machine, checking the required Java and Maven versions, GPG setup, Docker/Pulsar container steps, and lifecycle phases involved in clean install. Update the Getting Started documentation with the verified prerequisites and complete setup sequence, including all dependencies discovered during the fresh setup.

Written by the indexing model from the issue text.