docs(docs): fourth whole-repo pass over the non-API docs (#1103)

[JarryShaw]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly

• Put an x into all the boxes [ ] relevant to your pull request (like that [x])

• Use Preview tab to see how your pull request will actually look like

• 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. Narrow runs only, listed below.

• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible: N/A — centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

What is the purpose of your pull request?

Tick the commit type your subject line carries.

• fix — corrects a defect
• feat — adds a feature
• perf — changes performance, not behaviour
• refactor — changes neither behaviour nor performance
• test — tests only
• docs — documentation only
• ci — workflows or build tooling
• release — bumps the version or rolls up a distribution
• chore — anything else

The fourth whole-repo prose pass of #1103, over the hand-written docs pages, every index and toctree. It changes prose only: every changed .py file's AST is unchanged once string expressions are blanked, except where a docstring is added or vendor template text is edited together with its generated const file.

Accuracy:

• registry-protocol: a loud BaseError logs CRITICAL and installs the
  exception hooks; it no longer sets sys.tracebacklimit
• pep: only the docs build installs .[all,dev] on macOS; SCTP named as
  the one checksum verifier
• protocol-layer-placement: ARP and RARP have different EtherTypes
• extension-header-subclassing: IPsec is not already in the MRO
• engines index: PyPCAP's Python range matches its <3.12 marker
• releasing, mint-criterion: stale line citations fixed or dropped

Counts and timed context removed from workflows, ext, conf.py, the
layer-placement page, pep and the pcapkit index.

Clarity: the engine-gap exceptions on the overview, ext and pep pages,
the gate-only sentence and tier sentence in workflows, and grammar in
the API index pages; unresolved str and copyreg references fixed.

Refs #1103

[JarryShaw]

GOOD TO GO at 3a09a6e39. A Sonnet cross-review checked the reworded docs pages against the code they describe and found no counts, timed context or broken markup. The pinned docs build has no warnings.