Improve Zapier Documentation

[tejas7777]

Add a section on Zapier integration which does the following:

1. Maps the webhook fields to Zapier field names eg. appointment id to subject->Lesson id, service id to subject->Job Id etc.
2. Give a more end to end working example with steps to connect and debug (can be a simple workflow like - "Send an email whenever a tutor forgets to complete a lesson report for more than 24 hours"

This is from a feedback received from Reshmi Patel from Key Sounds UK