SIW concept doc: add image and update description

[brentschaus-okta]

Description:

• What's changed? SIW concept file.
• Is this PR related to a Monolith release? No.

Resolves:

• OKTA-1074060

Netlify Preview Link:

https://tbs-okta-1074060-update-siw-concept--dev-docs-preview.netlify.app/docs/concepts/sign-in-widget/

[okta-prod-github-app]

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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/sign-in-widget/index.md	94	100	85	94

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