docs: Build example Meltano project using `meltano-map-transform` with examples for common uses

Original description

The README for https://github.com/MeltanoLabs/meltano-map-transform lacks usage instructions, https://docs.meltano.com/concepts/plugins#mappers has an example of pipelinewise-transform-field but not meltano-map-transform, and I happen to know that some of the docs on https://sdk.meltano.com/en/latest/stream_maps.html apply to meltano-map-transform as well, but a user wouldn’t know that.

Since mapping in between extracting and loading is a form of transformation, I think we should document this functionality on https://docs.meltano.com/guide/transformation, and reference it from https://docs.meltano.com/guide/integration as well, as a new user wouldn’t know to find the relevant docs in https://docs.meltano.com/concepts/plugins#mappers. I also think it’s worth listing the mappers supported out of the box on https://hub.meltano.com/.

Proposal (from @aaronsteers):

1. Create a new entry in the example-projects repo with a set of example map transforms (hopefully covering all/most of what @sbalnojan outlines here).
2. Link to the new usage examples in:
   • the meltano-map-transform readme
   • the related SDK docs page
   • on docs.meltano.com

[tayloramurphy]

@sbalnojan thank you for updating with the context. @aaronsteers is that enough guidance to be able to assign to an engineer?

[sbalnojan]

I saw https://github.com/MeltanoLabs/meltano-map-transform already got an update making it way easier to understand. I personally think for now, having a good README will do the job. Some things I would love see are:

1. Minimum of three examples of the configuration
2. Examples of configuration should include the data going in and out (One row, before & after).
3. Example data should vary (a flat stream, maybe one with nested things,…)
4. Examples should vary in complexity AND try to hit the special cases… So
5. One example using just the built in functions
6. One example using “record.get”
7. One example using additional Python functions (like any short lambda function).
8. One of the examples should also use the type casting…
9. I like the one that’s already there referencing the “config”!
10. The README should mention an easy way to test my mapping (like using cat and or stdout,…).

Basically, we want a potential user, that is a first time meltano user, to have a bunch of examples he can simply modify to fit his purpose. Once we have a set of working examples, me or @afolson can take them and create additional tutorials etc. out of them.

@tayloramurphy