Broken links in the documentation

[kaarelmand]

In the main Table of Contents of the documentation, the link for '5. XASViewer' points you to https://xraypy.github.io/xraylarch/index.html, which does not exist. The same broken link can be seen in the equivalent 'previous topic' and 'next topic' entries in the adjacent chapters.

[maurov]

@kaarelmand as said in #408, any help is welcome! Feel free to propose a pull request for this.