Change psylow_subgroup to sylow_subgroup

[StevellM]

Related to renaming and consistency with the similar Oscar function: fixes #1015.

I have adapted the documentation to be as close as possible to the Oscar one, plus an example for free.

This would also require an Oscar counterpart where we remove the sylow_subgroup(::GrpAbFinGen) there (see https://github.com/oscar-system/Oscar.jl/blob/a7f9f88b91237001e8fcc467ff0e24892d641d41/src/Groups/GrpAb.jl#L285C1-L300C4)

[thofma]

Thanks!

[codecov]

Codecov Report

Merging #1355 (5c584db) into master (bf8c367) will decrease coverage by 0.05%.
The diff coverage is 80.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1355      +/-   ##
==========================================
- Coverage   70.29%   70.25%   -0.05%     
==========================================
  Files         351      351              
  Lines      111785   111785              
==========================================
- Hits        78578    78531      -47     
- Misses      33207    33254      +47     

Files	Coverage Δ
src/Deprecations.jl	100.00% <ø> (ø)
src/Grp/GenGrp.jl	69.38% <100.00%> (ø)
src/GrpAb/GrpAbFinGen.jl	77.98% <100.00%> (ø)
src/GrpAb/SubgroupEnum.jl	83.87% <100.00%> (ø)
src/GrpAb/stable_sub.jl	82.04% <100.00%> (ø)
ext/GAPExt/maximal_abelian_subextension.jl	0.00% <0.00%> (ø)

... and 26 files with indirect coverage changes

[fingolfin]

@StevellM can you edit the description of this PR to include something like "fixes #1015" or "resolves #1015" so that merging this PR will close that issue?

[lgoettgens]

This would also require an Oscar counterpart where we remove the sylow_subgroup(::GrpAbFinGen) there (see oscar-system/Oscar.jl@a7f9f88/src/Groups/GrpAb.jl#L285C1-L300C4)

I just added this change to oscar-system/Oscar.jl#3231