[DOCS] Make a Better Contributing Guide!

[TheCedarPrince]

It has recently dawned on me that although we do have a contributing guide, it could be better. Especially with respect to creating new functions for OCC, how to test out functions for OCC, and more. Generally speaking, I think a better way of outlining how to develop new OCC functions would be great. In particular, this process for creating new functions could look something like:

1. Download HealthSampleData
2. Run the Eunomia function and create a connection to the database using SQLite
3. GenerateTables but set export equal to true so one can have FunSQL representations of the OMOPCDM tables to play with
4. Link to the FunSQL tutorials
5. Walk through an example of how to build a new function
   1. Include tests
   2. Include dispatches
   3. Include documentation details
6. Finalize walk through with how to submit a PR

I think this would be great and could help more with onboarding as well as making our general process more clear. It uses a lot from the original tutorial here: https://juliahealth.org/OMOPCDMCohortCreator.jl/stable/tutorials/beginner_tutorial/

What do you think @Jay-sanjay and @Farreeda ? Am I missing anything general in this outline?

[Jay-sanjay]

As far as I see ,I guess there is nothing missing. Also we could add more if we felt would be needed in the future 👍