Update index.md - Writing exercise

[1510janu]

Part 1 - Writing

Rewritten the Okta Identity Engine overview page as part of the assessment.

Description:
What's changed?
I revised the content to enhance clarity, readability, and consistency while adhering to Okta's style guidelines. I also restructured the content for flow.
Updated to improve Acrolinx score.

Resolves:

• OKTA-XXXXXX

[acrolinxdevdocs-okta]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.

Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/concepts/oie-intro/index.md	77	43	61	81	97

• Scorecard links, by default, are only good for 7 days. Link for this commit's Content Analysis Report