feat: minor improvements to the new docstring blocks

[avik-pal]

• Makes the docstrings headers similar to how Documenter.jl renders them (without the # and makes the name clickable for link)
• adds support for CollapsedDocStrings meta. fixes support CollapsedDocStrings option from @meta block #180

[avik-pal]

Seems to be working: https://luxdl.github.io/DocumenterVitepress.jl/previews/PR181/api. I collapsed the docs just as a demo.

The reason for change 1 is mostly because https://lux.csail.mit.edu/dev/api/Lux/utilities#Lux.Training.apply_gradients! looks visually strange with > # ..... and the clickable area was too small

[avik-pal]

Created https://luxdl.github.io/DocumenterVitepress.jl/previews/PR181/internal_api as a demonstration and left the default for https://luxdl.github.io/DocumenterVitepress.jl/previews/PR181/api

[asinghvi17]

This looks good to me! I still don't like that the font size in docstrings is smaller than the main body, though - it's hard to read.

[lazarusA]

I like it as it is. Let's merge.