Add hideable sidebar and broken anchor checker to documentation

[FroggieFrog]

This PR enables a new feature for the documentation and also lets the build process fail, if there are any broken links or anchors inside the Markdown files.

• hideable side bar (https://docusaurus.io/docs/sidebar#hideable-sidebar)

  ... you can make the entire sidebar hideable, allowing users to better focus on the content. This is especially useful when content is consumed on medium-sized screens (e.g. tablets).

  

• broken anchors checker (https://docusaurus.io/blog/releases/3.1#broken-anchors-checker and https://docusaurus.io/docs/api/docusaurus-config#onBrokenAnchors)

  ... We recommend to turn it to throw and fail your CI builds instead of deploying broken anchors to productions.