Add instructions to update index mapping in developer guide

[vibrantvarun]

Is your feature request related to a problem?

Currently, if the developers want to introduce a new field in the index then they have to add the new mapping in certain files. This includes mapping files, entity class, test etc. If one forgets to add the mapping in mapping files then it will create silent failures and disrupt the logic.
Therefore, add documentation around this in developer guide.

[heemin32]

It would be better to implement programmatic safeguards so that any mistakes can be automatically detected.

[epugh]

I think the thrust of this is to understand where to start... I would like to add Name to Experiment, and I have no real idea how to do that. I can look at old PR's, but maybe a hint of how to do it would be helpful. I agree that programmatic safeguards also would be good.