docs/commands: add from.md, update subcommands (#1712)

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

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

Needs matching update for to*

Ref #1709

docs/commands/from-xml.md

@@ -1,8 +1,8 @@
-# from-xml
+# from xml
 
 Parse text as `.xml` and create table. Use this when nushell cannot dertermine the input file extension.
 
-Syntax: `from-xml`
+Syntax: `from xml`
 
 ## Examples
 
@@ -22,13 +22,13 @@ Let's say we've got a file in `xml` format but the file extension is different s
 </world>
 ```
 
-We can use `from-xml` to read the input like a `xml` file:
+We can use `from xml` to read the input like a `xml` file:
 
 ```shell
-> open world.txt | from-xml
+> open world.txt | from xml
 ━━━━━━━━━━━━━━━━
  world
 ────────────────
  [table 7 rows]
 ━━━━━━━━━━━━━━━━
-```
+```