Use the preferred git switch -c command

See https://www.infoq.com/news/2019/08/git-2-23-switch-restore/

Author: stevepiercy

docs/contributing/documentation/index.md

@@ -159,7 +159,7 @@ This section describes how to make contributions to files in the `plone/document
 1.  Create a new branch from `6.0`.
 
     ```shell
-    git checkout -b <new_branch>
+    git switch -c <new_branch>
     ```
 
 1.  Edit files, save, preview, and test.
@@ -247,7 +247,7 @@ We currently use the branches `plone/[email protected]`, `plone/plone.api@master
 1.  Create a new branch from the development branch.
 
     ```shell
-    git checkout -b <new_branch>
+    git switch -c <new_branch>
     ```
 
 1.  Make edits to files in `docs/<external_package>` using your favorite editor, and save, preview, and test.

docs/contributing/first-time.md

@@ -128,7 +128,7 @@ Then create a new branch from the default branch, and check it out to work on it
 We recommend using a branch name that includes the issue number and is descriptive of what it resolves.
 
 ```shell
-git checkout -b branch_name
+git switch -c branch_name
 ```
 
 Now you can edit your code without affecting the default branch.