docs/commands: add to.md, update subcommands (#1715)

This adds a top-level document for the new `to` command, with a list (of links) of all the subcommands.

All the to-* subcommands keep their filename, but the content is updated to use the new subcommand syntax.

Since not all subcommands have documentation, some items in the list are just text without a link. Also filled the list for the undocumented from* commands in the same style.

Fixes #1709

docs/commands/to-yaml.md

@@ -1,4 +1,4 @@
-# to-yaml
+# to yaml
 
 Converts table data into yaml text.
 
@@ -13,7 +13,7 @@ Converts table data into yaml text.
  1 │   │ filesystem │ /home/shaurya/Pictures
  2 │   │ filesystem │ /home/shaurya/Desktop
 ━━━┷━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━
-> shells | to-yaml
+> shells | to yaml
 ---
 - " ": X
   name: filesystem
@@ -33,7 +33,7 @@ Converts table data into yaml text.
 ────────────────────┼────────────────┼─────────────────┼───────┼─────────────────┼─────────────────
  Visual Studio 2017 │ [table: 1 row] │ [table: 5 rows] │       │ [table: 2 rows] │ [table: 2 rows]
 ━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━┷━━━━━━━┷━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━
-> open appveyor.yml | to-yaml
+> open appveyor.yml | to yaml
 ---
 image: Visual Studio 2017
 environment: