Avoid crash on private nested generic packages

[yrashk]

Problem

When private documentation is disabled, Process_Children still inspects a package's private declarations to recover dispatching operations. A nested generic package is neither handled nor skipped by that traversal, so GNATdoc reaches Raise_Not_Implemented and reports an internal error.

The failure is reproduced by:

package Private_Nested_Generic is
private
   generic
   package G is
   end G;
end Private_Nested_Generic;

Solution

Treat Ada_Generic_Package_Decl like the other private declarations omitted during public documentation generation. Public generic packages and runs using --generate-private retain their existing traversal behavior.

Add the reproducer to the executable nocrash fixture so the maintained testsuite guards the failure.

Verification

• Pristine GNATdoc 26.0.0 reproduced GNATDOC.NOT_IMPLEMENTED with GNAT 13.2.0, 14.2.0, 15.3.0, and 16.2.0.
• Patched GNATdoc 26.0.0 completed without the internal error and generated index.html in all four compiler lanes.
• The regression fixture fails with the pristine executable and passes with the patched executable.
• The patched GNATdoc 26.0.0 source build completed successfully.
• The maintained e3.testsuite suite passes: 31/31 tests.

[CLAassistant]

All committers have signed the CLA.

[godunko]

Thank you for contribution, pull request has been merged.