Add documentation on how to create a compiler plugin

[poorna2152]

Description

Add documentation on the steps to create a compiler plugin

[rajasurya-rjs]

hi i would love to do this. please assgin this to me

[gayaldassanayake]

Since this is a requirement for the internal developers, it was decided that it will be added under ballerina-platform/ballerina-lang@master/docs as a first step.

[gimantha]

This will be addressed in #9049