docs: fix stale references left by the API split (#184)

[caubut-charter]

What type of PR is this?

• documentation

What this PR does / why we need it:

Fixes stale references left behind by the API split. The repository used to define one monolithic
network-access-management API; #153 split it into network-access-devices and
network-access-domains, but several references to the pre-split world — and several documents
that had drifted from the tree — were never updated.

Old API name where a split API name is required

• TrustDomainsUser_Story.md said the API Provider must have implemented the "Network Access
  Management (NAM)" API. It is the Network Access Domains API, as the sibling
  device-registration story already states correctly.
• code/Test_definitions/README.md described a single API; the directory covers both.
• Five schema descriptions placed their resource "within the network access management system";
  each now names the API that actually exposes it.

Documented behaviour that does not exist

• TrustDomainsUser_Story.md listed 409 Conflict (INCOMPATIBLE_STATE) for Trust Domain deletion.
  deleteTrustDomain documents 204/400/401/403/404/500/503 and no 409 at all; the only 409 in that
  spec is ALREADY_EXISTS on creation. Replaced with the exception the spec actually defines.
• DeviceRebootRequest_User_Story.md omitted 409 ALREADY_EXISTS, 409 INCOMPATIBLE_STATE and
  422 MISSING_IDENTIFIER, all of which the devices spec does document. Added.

code/README.md drift

• The folder tree omitted Test_definitions/ and three of the four files in common/.
• The Component Breakdown table listed 5 of the 11 component files. It now lists all of them with a
  "Used by" column, produced by resolving every $ref transitively from both specs.
• NAM_Common.yaml's contents were described three different ways in three places, none of them
  complete. Reconciled against the file.
• Removed two redundant restatements so each component is described exactly once.

Reference fix — the root README pointed at a non-existent local APIBacklog.md; now links to
the file in camaraproject/APIBacklog.

Which issue(s) this PR fixes:

Fixes #184

Does this PR introduce a breaking change?

• Yes
• No

Special notes for reviewers:

• No CHANGELOG file is touched — released history. Four broken links there are reported below
  rather than edited.
• redocly lint: 0 errors, 9 warnings — unchanged from main.
• Merges cleanly with docs: document empty-list, 400/422, and GENERATE credential conventions (#129) #174. Conflicts with the [Trust Domains] Align documentation with CAMARA actors and terms #87 branch on one user-story row (made to converge,
  so the resolution is "take this branch's version") and with the Remove incomplete thread specification #65 branch on code/README.md.

Reported, deliberately not fixed here:

• CHANGELOG.md:30,31,32,57 — four links point at camaraproject/DeviceIdentifier /
  device-identifier.yaml instead of this repository, a bootstrap template copy-paste. Not edited:
  released history, and /CHANGELOG.md requires release-management review.
• Both specs reference an "API Readiness Checklist" document that does not exist anywhere under
  documentation/. Not edited: the sentence sits inside a CAMARA:MANDATORY block, so the fix
  belongs upstream in Commonalities or in actually adding the document.
• Two user stories link TMF640 at projects.tmforum.org, a retired Confluence instance, and
  documentation/API_documentation/README.MD:15 links a login-gated engage.tmforum.org download.
  Not changed — no verified replacement URLs.
• README.md:12 says the repository "does not yet belong to a CAMARA Sub Project" while lines
  49–50 refer to "this Sub Project" and the sp-nam mailing list. Likely CAMARA-managed
  boilerplate, so left alone.
• code/Test_definitions/README.md:111 — a lint recipe labelled "from anywhere" whose last
  argument is a repo-relative path, so it only works from the repository root.

Changelog input

release-note Corrected documentation left stale by the API split: pre-split API names in user stories and schema descriptions, a Trust Domain deletion exception documenting an undefined 409, missing reboot-request error responses, and an out-of-date component inventory in code/README.md.

Additional documentation

This section can be blank.

docs

[clundie-CL]

LGTM. The one note is that for you PR trio the merge order is #186 → #185 → #187. In view of #180, I think that becomes #180→ #186 → #185 → #187