Rename Index

The Index page and the `index.md` file seem to be hardcoded.

Renaming `index.md` crashes the app with:

```
FetchError: invalid json response body at http://localhost:4200/docs/index.json reason: Unexpected token < in JSON at position 0
at /path/to/addon/node_modules/node-fetch/lib/index.js:272:32
at processTicksAndRejections (internal/process/task_queues.js:97:5)
```

"Index" means an alphabetical list, a table of contents.

But the Index page typically contains an introduction to the addon and has no alphabetical lists.

I would like to be able to rename it.

PS If you agree with this and think this is "a good starting issue", please provide some guidelines, and I'll be happy to attempt a PR.