Reduce duplication in doctest setup

[fingolfin]

From docs/make.jl:

# We do *not* call `DocMeta.setdocmeta!` with value `:(using AbstractAlgebra)`
# because we want to force that each jldoctest has its individual
# `setup = :(using AbstractAlgebra)`.
# The reason is that the AbstractAlgebra doctests are run also in Nemo,
# where `using AbstractAlgebra` must be avoided, as a few symbols change
# their meaning, e.g. `QQ`.

As a result we are duplicating this line again and again in src which is rather annoying:

```jldoctest; setup = :(using AbstractAlgebra; AbstractAlgebra.set_current_module(@__MODULE__))

Ideally we would replace the comment in docs/make.jl by something like this:

DocMeta.setdocmeta!(AbstractAlgebra, :DocTestSetup, AbstractAlgebra.doctestsetup(); recursive = true)

But if we do that, then (at least in the past) the CI tests for Nemo, Hecke, Oscar may fail. This is because there this is done instead:

DocMeta.setdocmeta!(Nemo, :DocTestSetup, :(using Nemo); recursive = true)

Yet at least some of the AA doctests are also run by the Nemo/Hecke/Oscar CI. They now have a different setup block and as such, at least some of them run into test failures.

A potential fix might be to do something like this in Nemo (and then similar in Hecke, Oscar):

DocMeta.setdocmeta!(Nemo, :DocTestSetup, :(using Nemo); recursive = true)
DocMeta.setdocmeta!(Nemo.AbstractAlgebra, :DocTestSetup, Nemo.AbstractAlgebra.doctestsetup(); recursive = true)

But someone needs to experiment with this, once there is an AA releases with doctestsetup() available.

(And then we probably should also add Hecke.doctestsetup())