Layout change proposal

Hello!

I made some changes for the purposes of using this as a blog. I feel like these changes will help other people getting started for a multi-page site with a default global layout.

Notable
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* All HTML elements have their closing tags in the same file.
* head, header, and footer elements each have their own file respectively.
* html and body elements are part of the default layout.
* `index.md` is converted to Markdown.

Please don't merge without discussion. Thanks!