Added TOC support

[jdrowell]

Prerequisites

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• This pull request fixes a bug.
• This pull request adds a feature.
• This pull request introduces breaking change.

Description

Adds "Table of Contents" functionality for posts. I believe this was not implemented before, and I use it on my site.

Issues Resolved

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Checklist

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General

• Describe what changes are being made
• Explain why and how the changes were necessary and implemented respectively
• Reference issue with #<ISSUE_NO> if applicable

Resources

• If you have changed any SCSS code, run make release to regenerate all CSS files

Contributors

• Add yourself to CONTRIBUTORS.md if you aren't on it already