feature: Icon component

[jbmoelker]

To do

• Document icon setup, requires #35 to be merged first
• Decide if we need our own Icon wrapper to ensure the sprite version is always used

Changes

• Adds <Icon name="basename"> component, which inlines an SVG in src/icons/{basename}.svg using a sprite. This setup uses the astro-icon package, created and maintained by an Astro core member.
• Adds a dummy ShareButton component with a share icon to demonstrate the setup works.

Associated issue

Resolves #40

How to test

1. Open preview link
2. Verify the "share on social media" link at the bottom of the page has an icon
3. Verify the icon uses an icon sprite (with the DevTools element inspector)

Checklist

• [x] I have performed a self-review of my own code
• [x] I have made sure that my PR is easy to review (not too big, includes comments)
• I have added/updated tests to prove that my feature works (if not possible please explain why)
• [x] I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
• [x] I have added a decision log entry if the change affects the architecture or changes a significant technology
• [x] I have notified a reviewer

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