added sitemap icons to credits page (#1530)

_data/internal/credits/developer-activity.yml

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+---
+title: Developer Activity
+title-link: https://www.freepik.com/free-vector/developer-activity-concept-illustration_7230635.htm#page=1&query=Developer%20activity%20stories&position=1
+content: icon
+used-in: Sitemap
+artist: stories
+provider: Freepik
+provider-link: 'https://www.freepik.com/'
+image-url: /assets/images/sitemap/sitemap-planning.svg
+alt: 'Web Developer Working'
+type: icon
+---

_data/internal/credits/idea.yml

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+---
+title: Idea
+# this exact icon is no longer on noun project, this link is to the closest icon by the same artist
+title-link: https://thenounproject.com/search/?creator=1859235&q=idea&i=1366851
+content: icon
+used-in: Sitemap
+artist: Numero Uno
+provider: Noun Project
+provider-link: 'https://thenounproject.com/'
+image-url: /assets/images/sitemap/lightbulb.svg
+alt: 'Shining Lightbulb'
+type: icon
+---

_data/internal/credits/website.yml

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+---
+title: Website
+title-link: https://thenounproject.com/search/?creator=47612&q=website&i=3493536
+content: icon
+used-in: Sitemap
+artist: Clea Doltz
+provider: Noun Project
+provider-link: 'https://thenounproject.com/'
+image-url: /assets/images/sitemap/site.svg
+alt: 'Website Browser and Cursor'
+type: icon
+---