Add v5.1.0 / release-5.1 where it is missing

Related to fff8d90

CONTRIBUTING.md

@@ -34,6 +34,7 @@ Currently, we maintain six versions of TiDB documentation, each with a separate
 | Docs branch name | Version description |
 | :--- | :--- |
 | `master` branch | the latest development version |
+| `release-5.1` branch | the 5.1 stable version |
 | `release-5.0` branch | the 5.0 stable version |
 | `release-4.0` branch | the 4.0 stable version |
 | `release-3.1` branch | the 3.1 stable version |
@@ -51,6 +52,7 @@ Currently, we maintain six versions of TiDB documentation, each with a separate
 - If your changes apply to only one docs version, just submit a PR to the corresponding version branch.
 
 - If your changes apply to multiple docs versions, you don't have to submit a PR to each branch. Instead, after you submit your PR, trigger the sre-bot to submit a PR to other version branches by adding one or several of the following labels as needed. Once the current PR is merged, sre-bot will start to work.
+    - `needs-cherry-pick-5.1` label: sre-bot will submit a PR to the `release-5.1` branch.
     - `needs-cherry-pick-5.0` label: sre-bot will submit a PR to the `release-5.0` branch.
     - `needs-cherry-pick-4.0` label: sre-bot will submit a PR to the `release-4.0` branch.
     - `needs-cherry-pick-3.1` label: sre-bot will submit a PR to the `release-3.1` branch.

README.md

@@ -24,6 +24,7 @@ Currently, we maintain the following versions of TiDB documentation in different
 | Branch name | TiDB docs version |
 | :---------|:----------|
 | [`master`](https://github.com/pingcap/docs/tree/master) | The latest development version |
+| [`release-5.1`](https://github.com/pingcap/docs/tree/release-5.1) | 5.1 stable version |
 | [`release-5.0`](https://github.com/pingcap/docs/tree/release-5.0) | 5.0 stable version |
 | [`release-4.0`](https://github.com/pingcap/docs/tree/release-4.0) | 4.0 stable version |
 | [`release-3.1`](https://github.com/pingcap/docs/tree/release-3.1) | 3.1 stable version |