Updating contributing docs

[weerd]

We are using Testing Library on Phoenix, which is a great little utility library that helps with writing better and easier to understand tests for web UIs.

This utility library comes with a helper method for querying elements called getByTestId() which is very helpful but requires the data label to be data-testid. This is very close to our current standard but not exact, so it's useful to note that we want to switch to using new standard data-testid for testing moving forward!

This came up in a recent ghost inspector test fix.

If you want to see more examples in Phoenix for this data-testid in use, you can check the Buttons/ directory and see the different button tests which currently utilize this.

I also make the recommendation that we should not use data-id anymore for testing purposes. The Testing Library helper method only supports data-testid (of course the library itself supports querying by lots of different things, but setting data-testid as the standard seems good to provide consistency).

I also cleaned up this contrib doc a bit and added a section on sibling element spacing ✨

[DFurnes]

I like it! 🆔

[katiecrane]

I dig this!

[mendelB]

👏love it!