Just to comment on this point—
Actually, since the CPython docs is included as an intersphinx mapping in the devguide, its exactly as easy to link/cross-reference CPython docs targets in the devguide as in the CPython docs, as all the same syntax will work transparently.
Additionally, in the devguide cross-reference different specific versions of the CPython docs by adding them as separate intersphinx mappings (like we do in the PEPs), e.g. you can have a xref that always points to the latest dev version, the latest stable version, or a specific feature release.
And cross-referencing source files is as easy or easier thanks to the :cpy-file: role Ezio added to the devguide.