doc(overview): small reorganization of algebra/number theory (#5707)
doc(overview): small reorganization of algebra/number theory (#5707)

- adds Witt vectors
- adds perfection of a ring
- deduplicates Zariski topology
- moves some items to a new subsection "Number theory"





Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib