Update of website and documentation

[fipro78]

We recently migrated the NatTable Website from PHP to Hugo. In that process we moved the documentation from the website to the NatTable GitHub Wiki.

We hope that with this move it is easier to extend and update the documentation in the future.

[lppedd]

Moving NatTable to GitHub and its documentation to the Wiki seems the most logical choice! Thank you!

I think you could get rid of the NN_ prefix on each Wiki chapter.

[fipro78]

But then the chapters are sorted alphabetically on the right. At least if I understand it correctly. But there is of course room for improvement in that area.