Clean up main branch (#26)

* Removed `index.html` from `main`. Now it's auto-generated from
  `index.bs` and put into `gh-pages` branch. Available at https://wicg.github.io/unload-beacon/
* Fixed broken super-linter badge.

Related: #22

Author: mingyc

README.md

@@ -2,7 +2,7 @@
 
 Authors: [Darren Willis](https://github.com/darrenw), [Fergal Daly](https://github.com/fergald), [Ming-Ying Chung](https://github.com/mingyc) - Google
 
-[![Super-Linter](https://github.com/WICG/unload-beacon/workflows/linter.yml/badge.svg)](https://github.com/WICG/unload-beacon/actions/workflows/super-linter.yml)
+[![Super-Linter](https://github.com/WICG/unload-beacon/workflows/Lint%20Code%20Base/badge.svg)](https://github.com/WICG/unload-beacon/actions/workflows/super-linter.yml)
 [![Spec Prod](https://github.com/WICG/unload-beacon/actions/workflows/auto-publish.yml/badge.svg)](https://github.com/WICG/unload-beacon/actions/workflows/auto-publish.yml)