add introduction --> core-concepts/how-do-aeas-create-value

Proposed changes

Added how do AEAs create value section in introduction

Issues

Links to any issues resolved.

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.

[x] I have read the CONTRIBUTING document.
[x] 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.

If applicable

[ ] I have added tests that prove my fix is effective or that my feature works.
[ ] I have checked that code coverage does not decrease.
[ ] I have added/updated the documentations.
[ ] Dependent changes have been merged and published in downstream modules.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did, what alternatives you considered, etc.

fetchai / agents-aea

add introduction --> core-concepts/how-do-aeas-create-value #2846