chore(docs): refine the "getting started with modeling" page

[pavokta]

This is a first-half edit of a very, very long document.

I know that this is a lot to look over, so feel free to schedule me for a Zoom call so we can talk edits over before you merge.

Description

References

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [x] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [x] The correct base branch is being used, if not main
• [x] I have added tests to validate that the change in functionality is working as expected

[github-actions[bot]]

PR Preview Action v1.4.7 :---: :rocket: Deployed preview to https://openfga.github.io/openfga.dev/pr-preview/pr-656/ on branch gh-pages at 2024-03-13 18:33 UTC