Stop pretending OscarInterface has a manual

[fingolfin]

We might add one in the future, but for now there is none.

This may resolve (edit: it does not) #3967 but I am not sure as I don't fully understand what causes that one.

[lgoettgens]

The message still occurs in https://github.com/oscar-system/Oscar.jl/actions/runs/10074052750/job/27849342956?pr=3968#step:9:235, so #3967 doesn't get resolved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.09%. Comparing base (6724004) to head (41dba31).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3968   +/-   ##
=======================================
  Coverage   84.09%   84.09%           
=======================================
  Files         593      593           
  Lines       81744    81757   +13     
=======================================
+ Hits        68745    68757   +12     
- Misses      12999    13000    +1     

see 1 file with indirect coverage changes

[ThomasBreuer]

The proposed changes are a correction, the files claimed in the old version do not exist, and an empty list as PackageDoc value is allowed.