steamos-select-branch: add page (#16099)⁸

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/linux/steamos-select-branch.md

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+# steamos-select-branch
+
+> Select a SteamOS system release branch.
+> More information: <https://gitlab.com/users/evlaV/projects>.
+
+- View the current branch:
+
+`steamos-select-branch -c`
+
+- List all available branches:
+
+`steamos-select-branch -l`
+
+- Switch to the beta branch:
+
+`steamos-select-branch beta`
+
+- Switch back to the stable release:
+
+`steamos-select-branch stable`
+
+- Switch to the bleeding edge branch:
+
+`steamos-select-branch main`