Documentation website (including first Tutorial)

[michel-steuwer]

Added Tutorial app showing a basic overview of RISE and ELEVATE being used together.

This PR adds some additional syntax and functions to make the ELEVATE site a bit easier to understand (in the context of the tutorial).

There are also two bug fixes included:

• the default traversal order of ELEVATE programs now follows more closely the execution order making specifying traversals more intuitive
• a bug in the OpenMP vectorization code

[michel-steuwer]

This PR now also adds a documentation website, that is automatically deployed at: https://rise-lang.org/doc/

The process works as follows:

• Every markdown file that is added to the /docs/ folder corresponds to a page in the documentation
• Scala code that is embedded in these markdown files is checked to compile and run using mdoc (This is automatically run by the CI on every commit of a PR).
• The documentation website is powered by Docusaurus and configured in the /docs-website/ folder. The documentation page is deployed automatically on every successfully merged PR.
• The documentation website is hosted using GitHub pages on the gh-pages branch of the doc repository and publicly accessible at: https://rise-lang.org/doc/

[michel-steuwer]

I have integrated the content from #89 in the documentation.
@umazalakain I, therefore, suggest closing #89 in favour of this PR.

[Bastacyclop]

@michel-steuwer Its great to have a documentation website! maybe we can combine this with API docs auto-generated from our source code in the future? (so docs embedded in our code as well as code embedded in our docs)

[michel-steuwer]

@Bastacyclop thanks for the review. I have addressed all your comments. Could you have a final look and give your ok, or raise more points.