Autogenerate missing docs (#3514)

* Autogenerate missing docs

* Update ansi.md

* Rename question mark command docs

* Delete empty?.md

docs/commands/flatten.md

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+# flatten
+Flatten the table.
+
+## Usage
+```shell
+> flatten ...args {flags} 
+ ```
+
+## Parameters
+* ...args: optionally flatten data by column
+
+## Flags
+* -h, --help: Display this help message
+
+## Examples
+* flatten a table
+```shell
+> echo [[N, u, s, h, e, l, l]] | flatten | first
+ ```
+
+* flatten a column having a nested table
+```shell
+> echo [[origin, people]; [Ecuador, (echo [[name, meal]; ['Andres', 'arepa']])]] | flatten | get meal
+ ```
+
+  restrict the flattening by passing column names
+```shell
+> echo [[origin, crate, versions]; [World, (echo [[name]; ['nu-cli']]), ['0.21', '0.22']]] | flatten versions | last | get versions
+ ```
+