chore(docs-app): remove `document.write()` from docs `index.html`

Previously, the docs app used `document.write()`, causing the following
warning on Chrome:

```
A parser-blocking, cross site (i.e. different eTLD+1) script,
https://ajax.googleapis.com/ajax/libs/angularjs/1.7.9/angular.min.js, is
invoked via document.write. The network request for this script MAY be
blocked by the browser in this or a future page load due to poor network
connectivity.
```

In the past, `document.write()` seems to have been used in order for
browsers (such as Firefox) to work correctly with our dynamically set
`<base>` tag and relative style/script URLs.

This commit replaces `document.write()` with regular
`<style>`/`<script>` tabs to avoid the warning (and potential issues due
to poor network connectivity). It seems that the latest versions of
Chrome, Firefox and IE can handle this fine (without naticeable delays).

Fixes #15396