docs: link inventory-less package references through |name|_ substitutions #1936

The owner asked on #1928 for references to packages with no usable inventory (aenum, pcap, pcapfile, pyshark, and the dpkt/scapy/cryptography modules those inventories lack) to link to the project instead of rendering as plain literals. The suggested form is |name|_, with .. |name| replace:: ``name``` and .. name: . pcapkit/protocols/internet/esp.py:158anddocs/source/pcapkit/protocols/internet/esp.rst:166already use this form for|cryptography|`.

Plan:

• Define each substitution and target once, in rst_epilog in docs/source/conf.py.
• Use |pkg|_ for package- and module-level names.
• Keep member-level names as literals, since every one of them would link to the same homepage.
• Check whether docs(conf): resolve decorated overload aliases and qualify colliding third-party classes (#1917, #1918, #1920) #1929's format_class can emit the same substitutions in type hints.
• The pinned docs build must stay at 0 warnings. Watch for duplicate-target clashes with section titles such as "PyShark".
• Extend tests/project/test_docs_unresolvable_roles_1919_unit.py so that the inventory-less names link.

Blocked until #1928 and #1929 merge, because both rewrite the lines involved and #1929 owns conf.py.