Added new icon for dive deeper card

[mathewantony31]

Changes introduced

• Added new icon for the new dive deeper card on /help

How to test

To test, on the help repo, create a new folder with an index.md file with the following code:

---
title: "Dive Deeper"
description: Dive deeper into mapping concepts and Mapbox services.
helpCard: dive-deeper
navOrder: 2
layout: example
order: 1
hideSubpages: false # hide subpages from NavigationAccordion
contentType: tutorial
products:
- Documentation
# all OverviewHeader props are assigned in page-shell.js
overviewHeader: true
---

If it works correctly, when you go to http://localhost:8080/help, you should see a new "Dive Deeper" section with a book icon.

QA checklist

• [ ] No errors logged to console.
• [ ] Accessibility score in Chrome DevTools Audit/Lighthouse is 100% with Lighthouse version: #.#.#.
• [ ] Component is accessible at mobile-size viewport.
• [ ] Component is accessible at tablet-size viewport.
• [ ] Component is accessible at laptop-size viewport.
• [ ] Component is accessible at big-monitor-size viewport.

Open the test cases app locally on:

• [ ] Chrome, no errors logged to console.
• [ ] Firefox, no errors logged to console.
• [ ] Safari, no errors logged to console.
• [ ] Edge, no errors logged to console.
• [ ] Mobile Safari, no errors logged to console.
• [ ] Android Chrome, no errors logged to console.

Before merge

• [ ] Add entry to CHANGELOG.md to describe changes.
• [ ] If updating dependencies or creating a release, run npx browserslist@latest --update-db to update the browser data and commit any changes to package-lock.json.