docs(const): link the remaining internal references in the AppType and OptionType templates #1947

#1946 converts internal literals to roles everywhere except the AppType and OptionType templates and their generated const modules, which #1945 owns. These are left:

• TransportProtocol written as a literal: pcapkit/vendor/reg/apptype/apptype.py:476 and :1106, and pcapkit/const/reg/apptype/apptype.py:2522.
• AppType.get() written as a literal: pcapkit/vendor/reg/apptype/apptype.py:1006 and the generated pcapkit/const/reg/apptype/{tcp,udp,sctp,dccp}.py.
• AppType.__new__, __repr__, __str__, __int__, __eq__ and OptionType.__new__ have no docstrings. docs: link library-internal references and document the missing public classes (#1940) #1946 adds their entries on reg.rst and pcapng.rst, so they render empty until docstrings are written.

Each change goes in the vendor template, and the const modules are regenerated byte-identically.

Blocked until #1945 and #1946 both merge, because both edit these files.