Documentation: use https links

.gitlab/linters/check-cpp.py

docs/users_guide/bugs.rst

docs/users_guide/codegens.rst

docs/users_guide/debug-info.rst

docs/users_guide/extending_ghc.rst

docs/users_guide/exts/arrows.rst

docs/users_guide/exts/concurrent.rst

docs/users_guide/exts/data_kinds.rst

docs/users_guide/exts/defer_type_errors.rst

docs/users_guide/exts/gadt.rst

docs/users_guide/exts/generics.rst

docs/users_guide/exts/hiding_unexported.rst

docs/users_guide/exts/monomorphism.rst

docs/users_guide/exts/overloaded_strings.rst

docs/users_guide/exts/pattern_guards.rst

docs/users_guide/exts/poly_kinds.rst

docs/users_guide/exts/pragmas.rst

docs/users_guide/exts/quantified_constraints.rst

docs/users_guide/exts/recursive_do.rst

docs/users_guide/exts/roles.rst