apx: update page; apx-{pkgmanagers,stacks,subsystems}: add page (#10665)

* apx: update page; apx-{pkgmanagers,stacks,subsystems}: update page

* Apply suggestions from code review

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

* apx-stacks: update export description

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

---------

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

pages/linux/apx-stacks.md

@@ -0,0 +1,29 @@
+# apx stacks
+
+> Manage stacks in `apx`.
+> Note: user-created stack configurations are stored in `~/.local/share/apx/stacks`.
+> More information: <https://github.com/Vanilla-OS/apx>.
+
+- Interactively create a new stack configuration:
+
+`apx stacks new`
+
+- Interactively update a stack configuration:
+
+`apx stacks update {{name}}`
+
+- List all available stack configurations:
+
+`apx stacks list`
+
+- Remove a specified stack configuration:
+
+`apx stacks rm --name {{string}}`
+
+- Import a stack configuration:
+
+`apx stacks import --input {{path/to/stack.yml}}`
+
+- Export the stack configuration (Note: the output flag is optional, it is exported to the current working directory by default):
+
+`apx stacks export --name {{string}} --output {{path/to/output_file}}`