mathlib4 docs suggestions

Stream: general

[Winston Yin]

May I make three suggestions for mathlib4 docs:

1. Dark mode (or at least consistent with the mathlib3 docs, since they're under the same domain, and manual dark mode extensions can't distinguish the two sites)
2. Library directory on the left should not repeat the full address of files
3. Links from theorems and structures to their mathlib3 counterparts (through #align commands)

Thoughts?