Dedupe html & fix doc site previews

- Create template for generating multiple `index.html` files (root and `/docs/`)
- Add nunjucks dependency
- Append “(Preview)” to document title for easier identification of local preview tabs/windows (preview only)
- Add custom plugin to fix preview paths to `/docs` resources in markdown (preview only)
- Add front-matter plugin to `/docs/index.html` (missing in previous file, required for front-matter demo in docs)
- Add “build:html” npm script
- Fix preview paths to `/docs` resources in `index.html` files (preview only)
- Remove “Powered by docsify” from footer
- Remove “Vercel has given us a Pro account” from footer (acknowledgement provided via the “Special Thanks” section in README)
- Move “Edit Document” link from top-left to bottom-right of page
- Refactor package.json script (reorder)
- Refactor `.eslintignore` file (reorder and remove duplicate entries)