add introduction --> framework-application-areas

[FelixNicolaeBucsa]

Proposed changes

Added introduction section for application areas

Types of changes

What types of changes does your code introduce to agents-aea?
Put an x in the boxes that apply

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to stop working as expected)
• Something else (e.g. test, package, script, example, deployment, infrastructure, ...)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document.
• I have based my branch, and I am making a pull request against, the develop branch.
• Lint and unit tests pass locally with my changes.

[github-actions]

Visit the preview URL for this PR (updated for commit 6de3966):

https://fetch-docs-preview--pr2830-aea-introduction-fra-o0kygnjv.web.app

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