Add serialization info + NamespaceMap info

[bact]

What this PR do?

• Add a new subsection "4.4 Serialization information" to the chapter "Model and serializations"
• Use serialization and NamespaceMap content from https://github.com/spdx/spdx-3-model/blob/main/serialization/README.md
  • Revise it with standard terms like "may", "shall".
  • Update terms: spdxDocumentForArtfact -> serializedInArtifact, XCollection -> SpdxDocument

Background

• This is a required step to fix NamespaceMap: Remove TODO and fix caps spdx-3-model#796 (comment)
• Once this PR is merged and v3.0.1 is published: we can use this URL: https://spdx.github.io/spdx-spec/v3.0.1/serializations/ as a ref in NamespaceMap model description (instead of using a repo URL, which is not allowed in ISO doc)

What this PR still missing?

• Confirm if serializedInArtifact a correct relationship type for this paragraph:

  A Relationship of type serializedInArtifact can be used to relate an SpdxDocument to one more serialized for of the SpdxDocument.

• See JSON-LD Serialization doc: Fix relationship type name, Add validation info spdx-3-model#802 (comment)

[goneall]

LGTM - Thanks @bact

[maxhbr]

Looks good up to question

[bact]

We can now make NamespaceMap to refer to https://spdx.github.io/spdx-spec/v3.0.1/serializations/

• NamespaceMap: Remove TODO and fix caps spdx-3-model#796