feat: add Domain Management, completing the Domains API area

[mikemadeja]

Summary

• Adds Get-PiHoleDomain, New-PiHoleDomain, Update-PiHoleDomain, Remove-PiHoleDomain, covering Pi-hole's newer per-domain allow/deny API (GET/POST/PUT /domains/... + POST /domains:batchDelete). This is distinct from the existing List Management functions, which manage whole allow/block list subscriptions rather than individual domains.
• Single-item DELETE /domains/{type}/{kind}/{domain} is intentionally skipped in favor of the batch endpoint — same precedent already set by Remove-PiHoleList.
• Update-PiHoleDomain deliberately does not implement the API's documented "move a domain to a different Type/Kind via the same PUT" feature: live testing against a real server showed this leaves a stale duplicate at the original Type/Kind rather than actually moving it, so that path is left unimplemented rather than shipping broken behavior. Documented in the function's .DESCRIPTION.
• Adds a "Domain Management" category to tools/Update-ReadmeCommandReference.ps1 and tools/Update-ExampleOutput.ps1 (previously it would have silently folded into "Configuration & Diagnostics"), and regenerates README.md and docs/EXAMPLES.md accordingly.

This completes the "Domains" area from the API coverage audit (8 of 8 operations addressed: 7 implemented, 1 intentionally skipped).

Test plan

• All behavior verified directly against the real Pi-hole server before writing tests (create, duplicate detection, get/filter, update-merge, not-found error, batch remove, confirm-gone).
• 18 new Pester integration tests across the 4 functions, all passing against the real server.
• Confirmed server left in a clean state after every test run and after regenerating docs/EXAMPLES.md.
• Invoke-ScriptAnalyzer clean against the new functions.
• Full non-integration Pester unit suite passes (4/4; unaffected by this change).

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