Add more documentation for e2e tests on Gaia

[p-offtermatt]

Problem

We should have more documentation for how to work with the e2e tests for Gaia.

Closing criteria

There is more documentation for the e2e tests.

Problem details

We are missing documentation that makes it easy to work with the e2e tests.
We want people with little prior exposure to the Gaia repo itself to be able to add a new, simple test case in a reasonable time frame, and having good documentation would help.

I see two ways to tackle this:

1. High-level guidelines, akin to docs: Add guidelines for e2e tests interchain-security#1466
2. A step-by-step tutorial that goes through the motions of adding a simple test case

I think either or both could be useful.
In case of 2), it would be an extra bonus that the easiest way to write such a tutorial is probably to actually add a new test and take notes while doing it, so we'd get an extra test out of it.