add introduction --> what-is-the-aea-framework?

[FelixNicolaeBucsa]

Changed introduction by adding first section: "What is the AEA Framework?" @5A11 @GataKamsky

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 933d546):

https://fetch-docs-preview--pr2809-aea-introduction-wha-g5ltzmbb.web.app

_{(expires Fri, 27 Jan 2023 10:46:08 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: f2de39fd4e81249941960b74fbab0a62d90d69f8}