doc: [MER-2380][MER-2381] add manpages for av stack next and prev (#158)

Co-authored-by: aviator-app[bot] <48659329+aviator-app[bot]@users.noreply.github.com>

docs/av-stack-next.1.md

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+# av-stack-next 1 "" av-cli "Aviator CLI User Manual"
+
+# NAME
+
+av-stack-next - Checkout a later branch in the stack.
+
+# SYNOPSIS
+
+`av stack next [<n> | --last]`
+
+# DESCRIPTION
+
+Checkout a later branch in the stack. Without any options, this will default to checking out the next branch in the stack.
+
+# OPTIONS
+
+`<n>`
+: Checkout to the branch that is `<n>` branches after the current branch in the stack.
+
+`--first`
+: Checkout to the last branch in the stack.
+
+# SEE ALSO
+
+`av-pr-prev`(1)

docs/av-stack-prev.1.md

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+# av-stack-prev 1 "" av-cli "Aviator CLI User Manual"
+
+# NAME
+
+av-stack-prev - Checkout a previous branch in the stack.
+
+# SYNOPSIS
+
+`av stack prev [<n> | --first]`
+
+# DESCRIPTION
+
+Checkout a previous branch in the stack. Without any options, this will default to checking out the previous branch in the stack.
+
+# OPTIONS
+
+`<n>`
+: Checkout to the branch that is `<n>` branches before the current branch in the stack.
+
+`--first`
+: Checkout to the first branch in the stack.
+
+# SEE ALSO
+
+`av-pr-next`(1)