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
### After
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
