docs(stacks): add checkout command and --base option

src/content/docs/stacks/creating.mdx

@@ -23,6 +23,12 @@ This is equivalent to `git checkout -b feat/my-feature --track origin/main`. You
 can also create branches however you normally do. Stacks doesn't require its own
 branch creation command.
 
+To branch from something other than `main` (a release branch, for example):
+
+```bash
+mergify stack new feat/my-feature --base origin/release-v2
+```
+
 ## Make Commits
 
 Work normally. Each commit should represent one logical unit of work, something

src/content/docs/stacks/setup.mdx

@@ -40,9 +40,19 @@ This installs:
 - A `pre-push` hook that warns you if you accidentally use `git push` instead
   of `mergify stack push`.
 
-- A [Claude Code](https://claude.ai/code) skill that teaches the AI
-  coding agent how to work with Mergify Stacks, so it automatically uses
-  `mergify stack push` instead of `git push`.
+## Claude Code Integration
+
+If you use [Claude Code](https://claude.ai/code), add the Mergify marketplace
+and install the Stacks plugin:
+
+```bash
+/plugin marketplace add mergifyio/mergify-cli
+/plugin install mergify-stack
+```
+
+This teaches Claude Code how to work with Mergify Stacks, so it automatically
+uses `mergify stack push` instead of `git push` and follows the stacked PR
+workflow.
 
 ## Verify It Works
 

src/content/docs/stacks/team.mdx

@@ -41,6 +41,19 @@ to install anything.
 
 As more developers see the benefits in review, adoption spreads naturally.
 
+## Working on Someone Else's Stack
+
+If a teammate has pushed a stack and you want to pick it up locally (to help,
+pair, or continue the work), use `checkout` with their branch name:
+
+```bash
+mergify stack checkout --branch feat/their-feature
+```
+
+This reconstructs the stack locally by fetching the remote branches and
+rebuilding the commit chain. You can then make changes and push updates with
+`mergify stack push` as usual.
+
 ## When to Use Stacks
 
 Not every change needs a stack. Use stacks when the alternative is a large,