Allow custom nav / sidebar in MarkdownVitepress

[avik-pal]

One of the reasons I keep the whole config mts around in Lux is to allow me to customize the nav and sidebar but that means I have to periodically do LuxDL/Lux.jl#1250 which is not ideal. For now, if we allows these options to be list that simply gets inserted into the config file that would be good enough.

[lazarusA]

what syntax would you imagine for this?

[avik-pal]

Just a simple

MarkdownVitepress(;
   nav = ...,
   sidebar = ...
)

Then we replace the corresponding ones in the config file automatically without constructing our own nav/sidebar