[Solo] Add anchor link to content headings

[evanwon]

This change adds anchor links to post headings, enabling you to share deep links to your articles so that people can navigate directly to the relevant content you're trying to share. The anchor link is hidden until you hover over (or touch) the heading, and once it's displayed you can copy the link to share it with someone else.

Here's a demo of what it looks like:

solo-heading-anchor-demo.mp4

And I'm hosting a live demo on my personal website which you can see here: https://evanw.com/2021-my-year-in-reading/#the-stats

This feature is really useful when you write longer-form content and want to share a specific section with someone, or if you're trying to link to specific sections within your own content.

If there are any questions or concerns about this change, please let me know. Thanks for the consideration!