use DocumenterVitepress.jl for documentation

[CarloLucibello]

It would we nice to have a right pane in doc pages
https://fluxml.ai/Flux.jl/dev/reference/models/layers/
so that one can see all layers at a glance and quickly jump to any one of them

With DocumenterVitepress.jl we would have a nice right pane
https://luxdl.github.io/DocumenterVitepress.jl

Discussion of right pane for Documenter.jl is in
JuliaDocs/Documenter.jl#1688

Alternatives are

• the creation of an @index https://documenter.juliadocs.org/stable/man/syntax/#@index-block
• making the docstring collapsed by default setting CollapsedDocStrings = true
  https://documenter.juliadocs.org/dev/man/syntax/#@meta-block

[mcabbott]

One vote against.

If the page listing layers is really too hard to navigate, it can be split up, or given a table of contents. But I'm not convinced it is.