[docs] Improve documentation about pipeline config

[gkulin]

Configure a pipeline and push updates should be clearer that you have to create the azure-dev.yaml yourself if it doesn't already exist. It should also mention the new features with pipeline variables.

We can also improve the Configure a CI/CD pipeline for your template unit in azd's training module

[jongio]

@gkulin With #4030 we now ask the user to add the default azure-dev.yml file if they don't already have it. should we close this issue?

[pamelafox]

Can you adjust that error message to also look for .yaml? It error'ed on me, presumably because I use .yaml, but apparently .yaml is the preferred extension from the YAML creator, so that's what I've been using.

Alternatively, I change all my repos to yml.

[gkulin]

closed via https://github.com/MicrosoftDocs/azure-dev-docs-pr/pull/5814

will open another issue for the error message