add introduction --> core-concepts/framework-design

Proposed changes

Added introduction section --> core concepts: framework design and architecture

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/framework-design #2834