In-process vs out-of-process extensibility

[jhadobe]

This PR modifies the index.md page to explain the out-of-process extensibility offerings for Adobe Commerce and contrast them with the in-process offerings provided by both Adobe Commerce and Magento Open Source.

Staging: https://adobedocs.github.io/commerce-extensibility/

Additionally, this PR adds a Learning Path page to provide a list of resources along with their context for developers.

Staging: https://adobedocs.github.io/commerce-extensibility/app-development/learning-path/

[keharper]

You're definitely missing a discussion on App Builder. The first use case for eventing required App Builder. However, you can now use eventing without App Builder when you set up webhooks. The Admin UI SDK requires it.

[keharper]

I prefer the slide be replaced with an SVG file. Not a blocker.

[jhadobe]

Not publishing this until after @cpartica and @slamech have reviewed.