docs(rc3): record the v2.1.0-RC.3 tag and GitHub Release, and the missing container image for the v2.1 tags

The tag is annotated on d592442 and the release is published as a pre-release. Also recorded as open item 7: the server Dockerfile is net10.0-only and does not copy global.json, so the v2.1 line ships without a container image and the v2.1 tags are excluded from Docker and from the net10 CI/CD Pipeline instead of leaving red runs on every tag.

Author: MPCoreDeveloper

docs/2.1.0-RC.3_WHAT_CHANGED.md

@@ -261,6 +261,13 @@ artifacts for the shipped default; the throughput band moves with machine load.
 6. **Single-file (`.scdb`) migration is one-way** for v1.x files (unchanged from v2.0, `.backup`
    preserved), and there is **no downgrade path** for the new inline-capacity layout: an older build
    reading a new file would misread its inline slots.
+7. **No container image for the v2.1 line yet.** `src/SharpCoreDB.Server/Dockerfile` is `net10.0`-only
+   (`sdk:10.0` / `aspnet:10.0`) and does not copy `global.json`, so it cannot build this branch's
+   `net11.0` projects. `v2.1*` tags are therefore **excluded** from the Docker workflow (and from the net10
+   `CI/CD Pipeline`, whose pack/publish jobs are master-only anyway) rather than leaving a red run on every
+   tag; a net11-aware Dockerfile is the follow-up. Until it exists the RC ships **without** a
+   `ghcr.io/mpcoredeveloper/sharpcoredb-server:2.1.0-RC.3` image — the Aspire package's example still
+   points at the published `2.0.0.2` image tag.
 
 ---
 
@@ -322,4 +329,10 @@ as the `sharpcoredb-server` executable and container image (§6).
 never pushed, so RC.3 is the first release of the v2.1 line to carry `SharpCoreDB.Search`,
 `SharpCoreDB.HybridSearch` and `SharpCoreDB.Aspire.Hosting`.
 
+**Git tag and GitHub Release:** [`v2.1.0-RC.3`](https://github.com/MPCoreDeveloper/SharpCoreDB/releases/tag/v2.1.0-RC.3)
+— an annotated tag on `d5924420`, published as a pre-release with the release notes. The tag deliberately
+triggers neither the net10 `CI/CD Pipeline` (its SDK cannot read this branch's `.NET 11` `global.json`, and
+its pack/publish jobs are master-only) nor `Docker` (its Dockerfile is `net10.0`-only): both workflows now
+exclude `v2.1*` tags, and the reason is carried inline in each file. See §5 item 7.
+
 ---