HTML Include Plugin

[thescientist13]

Related Issue

resolves #788

> _Note: [this issue will manifest in this PR](https://github.com//pull/712#issuecomment-945019172) until #782 is merged_

Summary of Changes

1. Created a new plugin to reflect the design derived from this spike (see below details)
2. Updated website documentation and implemented the custom element variant for the website footer

Interesting observation about the footer here

1. Spiking out the idea of what a plugin might look like for supporting the old deprecated standard of [HTML includes](https://github.com/whatwg/html/issues/2791) (not to be confused with [HTML modules / imports](https://github.com/WICG/webcomponents/issues/645))

TODO

Going further, would like to see how far we can take it if we were to support something like this (notice the .js extension)

<link rel="html" href="/includes/html-include-test.html.js"></link>

const getIncludeTemplate = async (data) => {
  const { name } = data;
  
  return `Hello ${name || 'World'}! 👋`;
}

export { getIncludeTemplate };

Then the plugin could just import from that file and statically stamp out the contents that way? 🤔

[thescientist13]

Interesting observation but I noticed while working on this PR that on the main site, the (font?) size of the footer is slightly smaller on the home page vs any of the other pages / templates. Not sure if just me?

cc: @aholtzman