Plainify flexsearch index values

[schnerring]

Currently, the flexsearch index is populated with the non-planified .Content page variable.

You can try searching for the following and be able to find results: <div class="alert alert-warning d-flex" which is not part of the actual content but rather rendered HTML.

Using .Plain fixes this:

.Plain
the Page content stripped of HTML tags and presented as a string.
— https://gohugo.io/variables/page/

I inserted the following in content/en/docs/prolouge/commands.md to test whether you can search for HTML tags that are part of the actual content:

### styles

Check styles for errors:

```html
npm run lint:styles [-- --fix]
<div class="test">foobar</div>
```

And it works:

• <div class="test">foo returns a search result
• <div class="alert alert-warning d-flex" doesn't return search results

On top of that the resulting index.js is way smaller:

• development: 32.2 kB -> 15.3 kB (~27% smaller)
• production: 15.3 kB -> 7 kB (~54% smaller)

[h-enk]

Wow! Thanks, thanks, thanks. Verified (https://deploy-preview-511--doks.netlify.app/).

[h-enk]

Since your deep in FlexSearch now 😉 , do you have any ideas for #514?