Closes T2379

hat tip to @rdicosmo for noticing

Hinting at the origin_visit_update model

Related to T2310

they will remain for internal use only, so should not be mentioned in this
public spec

this reverts the documentation part of 67fade5f

related to T1330

used by the swh-docs to generate images and schemas when building the
whole documentation.

to prevent sphinx from complaining about these not being included.

double back-quote (``) are not interpreted when inside a emphasis block.

Promote "Resolution" to a new top-level section, because it's now rather
voluminous.

As part of this document the resolver limitations of identifiers.org, that get
in the way of PID contextual information.

Closes T1015
Closes T1223

it makes life a bit easier for CI (less dependencies).

Related D454

text adapted from iPRES 2017 paper about Software Heritage, section "Data
Model"

Closes T787

Closes T1041

Closes: T335

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.