[website] Bump Docusaurus to 3.1.1 and fix broken anchors

The website build has started failing for an unknown reason, but there
was a warning about the Algolia integration and bumping Docusaurus seems
to fix the issue. Docusaurus 3.1.1 introduces checks for broken anchors
which is also fixed in this PR. One of the broken anchors was to a
section on FixedPoint, as FixedPoint was removed, this section was
deleted.