[guides] Add a new guide about the MemorySheet (#2532)

Co-authored-by: anjakefala <[email protected]>

visidata/guides/MemorySheet.md

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+# The Memory Sheet 
+
+The **MemorySheet** provides a place to manually save cell values for later reference. It also automatically stores clipboard data when rows, cells or columns are copied.
+
+- {help.commands.memo_cell}
+- {help.commands.memo_aggregate}
+- {help.commands.open_memos}