add AGENTS.md with AEM Edge Delivery project guidelines/instructions

[shsteimer]

Please always provide the GitHub issue(s) your PR is for, as well as test URLs where your change can be observed (before and after):

Fix #

Test URLs:

• Before: https://main--aem-boilerplate--adobe.aem.live/
• After: https://agents-md--aem-boilerplate--adobe.aem.live/

[aem-code-sync[bot]]

Hello, I'm the AEM Code Sync Bot and I will run some actions to deploy your branch and validate page speed. In case there are problems, just click a checkbox below to rerun the respective action.

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[aem-code-sync[bot]]

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