Add some example CSS

## What does this MR do?

- add some minimalist CSS
- show an example of how to correctly reference CSS

## Are there points in the code the reviewer needs to double check?

Not really...

## Why was this MR needed?

The README mentions wrongly referencing CSS in Troubleshooting but does not show an example of how to do it right.

## What are the relevant issue numbers?

None.

## Screenshots

## Before

![before](/uploads/475480ff7d9326bc32f030a4762ba94d/before.png)

## After

![after](/uploads/0a49104df21ca3c52a05ac346411c034/after.png)

See merge request !12

Use valid HTML in the example

## What does this MR do?

Add a bit more HTML to the example to make it valid HTML5.

## Are there points in the code the reviewer needs to double check?

Nope.

## Why was this MR needed?

The current example is not valid: https://validator.w3.org/nu/?doc=https%3A%2F%2Fpages.gitlab.io%2Fplain-html%2F

## What are the relevant issue numbers?

None.

## Screenshots

### Before

![invalid](/uploads/3df7abeff8e8c2f51a00d9a697c75065/invalid.png)

### After

![valid](/uploads/c38fc2be423ae1fd17ae87efd24936b7/valid.png)


See merge request !11

Move index.html to simplify .gitlab-ci.yml

## What does this MR do?

- move `index.html` to public folder
- simplify `.gitlab-ci.yml`

## Are there points in the code the reviewer needs to double check?

Not really...

## Why was this MR needed?

Every build moves the `index.html` to the public folder—which is unnecessary if it can be there from the beginning.

## What are the relevant issue numbers?

None.


See merge request !10

Use smaller Docker image to speed up build

See merge request !9