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

[FelixNicolaeBucsa]

Proposed changes

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

Issues

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Types of changes

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• 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

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• 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.

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

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Visit the preview URL for this PR (updated for commit acacd6c):

https://fetch-docs-preview--pr2834-aea-introduction-cor-r4b2l2yn.web.app

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