Create docs for reclaiming an index

Related to deprecation of the wiki https://github.com/paritytech/wiki/pull/335

Old:

To reclaim an index I, you must be sure that I is currently mapped to a dead account (i.e. one with zero total balance). You need to craft a transaction that creates a new account by placing some balance B in it. B must satisfy these conditions:

B % 256 == 105

B / 256 % E == I

Where E is the quantized account index count:

E := (next_set_index * enum_set_size / 256 + 1) * 256

Given an amount to transfer A, you can calculate the lowest amount that is at least A, A' which will attempt to reclaim I as:

(Q(N / 256 + (E - 1 - I), E) + I) * 256 + N % 256

Where

N := Q(A + (256 - 1 - 105), 256) + 105

[joepetrowski]

Could potentially just live with the Indices module reference docs

[shawntabrizi]

@JoshOrndorff I think maintenance is handed off to the core dev team, but we are still on the hook for doing the first draft and getting it reviewed and merged in. But that could be different depending on who you ask so… ¯\(ツ)/¯