Move development documentation under docs/development

The Development section holds more than one page and will grow further,
so give it its own directory. development.md becomes development/index.md,
which keeps its published URL, and the custom CLI command page moves
alongside it.

Update the nav, the cross-link between the two pages, and the README
link to the custom command documentation, which is now published at
/development/custom_commands/.

docs/development/custom_commands.md

@@ -1,7 +1,7 @@
 # Custom CLI Commands
 
 MVT can load additional top-level commands into `mvt-ios` and `mvt-android`.
-Custom commands are different from [custom forensic modules](development.md#custom-modules):
+Custom commands are different from [custom forensic modules](index.md#custom-modules):
 commands add new CLI operations, while modules add analysis steps to existing
 `check-*` commands.