docs: link inventory-less packages through rst_epilog substitutions (#1936)

[JarryShaw]

• Searched for similar pull requests

• Followed the coding style (make pylint, make mypy, make isort)

• make test passes, and a test case covers the change

• Added a changelog entry — N/A — centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

• docs — documentation only

As ruled on #1928, every third-party project substitution and link target is now defined once, in rst_epilog in docs/source/conf.py. The per-page and per-docstring copies are removed. Package mentions of aenum, cryptography, dpkt, emoji, pcapfile and pyshark are now |pkg|_ links.

Engine, adapter and toolkit names, requirement lists and member-level names stay literals. pcap also stays a literal, because both PyPCAP and pcap-ct install it, so no single project page fits. format_class is unchanged: it names classes, and signatures do not expand substitutions.

The new tests/project/test_docs_epilog_links_1936_unit.py pins the epilog, the links and the absence of duplicate definitions. It fails on main. The pinned docs build reports 0 warnings.

Closes #1936

[JarryShaw]

GOOD TO GO at 1ca8b55c9. A Sonnet cross-review rescanned all of pcapkit/ and docs/source/ with its own script. Every |x|_ in use has exactly one definition, in rst_epilog, and every removed per-file URL matches its epilog replacement except for pcap-ct's trailing slash. Its own pinned build of 198 pages has 0 warnings and no duplicate-target messages, and no raw |x|_ appears outside _modules/. The links render on the index, internet, 3rdparty and enum pages. All changed .py files are AST-identical, the new test fails on main, and merge-tree with #1927 is clean. A few package names stay literals inside engine names or requirement phrases (pycrate, requests), which matches the stated rule.