[3.1] Add JSON-LD serialization annotations link

[bact]

As agreed in 23 June 2026 Tech call,
add JSON-LD serialization annotations URL and its description
to the "Serialization in SPDX 3 JSON" section.

To fix spdx/spdx-3-model#1381

Note

Within the "Serialization in SPDX 3 JSON" section,
to have three related subsections about serialization mechanics sit together for better flow,
this PR swap position of "JSON-LD context file" and "SpdxDocument":

Current:

1. A strict subset of JSON-LD
2. JSON-LD context file
3. SpdxDocument
4. JSON-LD validation
5. Examples

Proposed in this PR:

1. A strict subset of JSON-LD
2. SpdxDocument <---- Was (3)
3. JSON-LD serialization annotations <---- NEW
4. JSON-LD context file <---- Was (2)
5. JSON-LD validation
6. Examples

[bact]

From Tech call 30 June 2026 (from @henrylyons's presentation):

• Document what annotations.ttl contains and when an implementer must consume it

Should put that in this PR too

Update 7 July 2026: Done

[goneall]

LGTM

[goneall]

Discussed on the 21 July 2026 tech call - changes look OK, need one more reviewer.

@zvr - do you want to do a review and merge if OK?

[goneall]

@bact - looks like we got 2 reviews - could you resolve the merge conflict and merge?