Add documentation for FenwickTree

[eulerkochy]

With #485 in the master, there is a need for documenting this DS.

[kmsquire]

Whoops... should have asked you to do that before merging. Oh, well, if you could add some documentation, that would be great--thanks!

[eulerkochy]

Ok, cool!

[sambitdash]

I feel the data structure should be called "BinaryIndexedTree" or something similar. In general the package contains very few DS that are named after the inventor. @eulerkochy and @kmsquire, please suggest if that change is useful.

[eulerkochy]

Why not honour the inventor when there is a scope of doing so! imo, it hardly would matter if we rename it, as both terms are quite interchangebly, plus it has been mentioned in the implementation of the DS.

struct FenwickTree{T}
    bi_tree::Vector{T} #bi_tree is shorthand for Binary Indexed Tree, an alternative name for Fenwick Tree
    n::Int
end