Add a docs-spellcheck linter

Created by: coliff

I love that this project includes many tests and linters meaning that any broken links, invalid HTML or CSS/JavaScript bugs are caught before a PR can be made.

I think it'd be helpful to add a docs-spellcheck linter so that any spelling errors are caught before being merged. I propose to use the popular cSpell project as it is customizable, easy to use, has useful built-in dictionaries and has a VS Code extension, GitHub Action and CLI options.

There are some Bootstrap terms/words in the docs which show as false positives so we can add a custom dictionary which is automatically read if file is called .cspell.json (cspell.json also works but having a . at the beginning keeps all the configs together). That file would look something like this:

{
  "version": "0.2",
  "words": [
    "affordance",
    "allowfullscreen",
    "autohide",
    "autohiding",
    "autoplay",
    "autoplays",
    "blazingly",
    "Blockquotes",
    "Bootstrappers",
    "borderless",
    "browserslist",
    "browserslistrc",
    "btncheck",
    "btnradio",
    "callout",
    "callouts",
    "clearfix",
    "Codesniffer",
    "colorspace",
    "combinator",
    "Contentful",
    "Cpath",
    "Crossfade",
    "cssgrid",
    "Csvg",
    "Datalists",
    "Deque",
    "discoverability",
    "docsearch",
    "docsref",
    "dropend",
    "dropleft",
    "dropright",
    "dropstart",
    "dropup",
    "errorf",
    "favicon",
    "favicons",
    "flexbox",
    "fullscreen",
    "fusv",
    "getbootstrap",
    "globby",
    "Grayscale",
    "Hoverable",
    "hreflang",
    "hstack",
    "jsdelivr",
    "Jumpstart",
    "keyframes",
    "libera",
    "Libsass",
    "lightboxes",
    "Lowercased",
    "markdownify",
    "mediaqueries",
    "minifiers",
    "monospace",
    "mouseleave",
    "navbars",
    "navs",
    "Neue",
    "noindex",
    "Noto",
    "offcanvas",
    "popperjs",
    "prebuild",
    "precompiled",
    "prefersreducedmotion",
    "prepended",
    "printf",
    "rects",
    "relref",
    "rgba",
    "roboto",
    "RTLCSS",
    "ruleset",
    "screenreaders",
    "scrollbars",
    "scrollspy",
    "Segoe",
    "sirv",
    "srcset",
    "Stylelint",
    "subnav",
    "svgo",
    "tabbable",
    "textareas",
    "toggleable",
    "touchend",
    "twbs",
    "unitless",
    "Unported",
    "unstylable",
    "unstyled",
    "Uppercased",
    "urlize",
    "vbtn",
    "viewports",
    "vstack",
    "walkthroughs",
    "WCAG",
    "webfont",
    "zindex",
    "بالعالم",
    "مرحبا"
  ],
  "language": "en,en-US",
  "ignorePaths": [
    ".cspell.json"
  ]
}

We can install cSpell CLI by adding "cspell": "^5.9.0", to devDependencies and then in the scripts in package.json we can add something like: "docs-spellcheck": "cspell \"site/**/*.md\"", (I set it to scan just markdown files and not the examples HTMLs so the Arabic/RTL files are ignored).

Alternatively the spellcheck could just be done via GitHub Action - or just ask everyone contributing to install this extension for real-time spellcheck in VS Code. :-) https://marketplace.visualstudio.com/items?itemName=streetsidesoftware.code-spell-checker