docs: shipped modules with no documentation page (const.linux, vendor.tls, vendor.sslv2 and the checksum packages) #2559

From the 2026-11-01 conventions audit, re-verified on 73add05a3. documentation.rst, "Resolvable Targets": every importable shipped module is documented, with NotImplemented/ the one exception.

• No module, currentmodule or automodule directive under docs/source names: pcapkit.const.linux and .arphrd; pcapkit.vendor.tls and .content_type, .handshake_type; pcapkit.vendor.sslv2 and .handshake_type, .version; and the packages pcapkit.protocols.misc.checksum, pcapkit.protocols.data.misc.checksum, pcapkit.protocols.schema.misc.checksum.
• The counterparts exist: docs/source/pcapkit/vendor/linux.rst, const/tls.rst, const/sslv2.rst. docs/source/pcapkit/protocols/misc/checksum.rst documents the three ip_pseudo submodules (misc, schema, data) but not the three packages.
• Probe: walk pcapkit/ for modules (skip NotImplemented), diff against the directive names; the difference is exactly the modules above.

Fix: add const/linux.rst, vendor/tls.rst and vendor/sslv2.rst mirroring their counterparts, register them in their toctrees, add the three package directives to misc/checksum.rst, and re-run the probe.
Blocked: #2543 edits misc/checksum.rst (it adds a checksum submodule there); unblocks when it merges (gh pr view 2543 --json state -q .state prints MERGED). The other pages do not depend on it.