SC-19140 Update the Synthetics CI/CD GitHub Action and its documentation

[HSSalman]

This PR updates the Synthetics CI/CD GitHub Action and its documentation.

Aside from improving the overall documentation tied to the feature, the actual logic of the code was improved in order to make working with it easier. The previous hardcoded values in the middle of the code were replaced with clearly defined variables, with associated descriptions being added to explain why these are needed and where to find them. I also updated the dependency we were using to avoid the warning it was previously generating (Node 16 no longer being supported), and replaced the screenshots to reflect the current GitHub UI.

I tested the new workflow with a few different cases:

• Monitor passing
• Monitor failing
• Some error unrelated to the monitor itself, such not specifying the API key correctly (wasn't being handled well by the previous logic)

Note that we should also update the code and docs for the other CI/CD integrations listed here, but for now I've focused only on GitHub actions, as that's what we're working towards at the moment.

[HSSalman]

Made another series of changes to make it easier for people to set this up or modify it and added the docs to go along with these changes. This also lets users make use of this on the EU region, whereas it was previously hardcoded to be on US only.