Add some detail to the Standard Library doc (nushell#1546)

Author: NotTheDr01ds

book/standard_library.md

@@ -1,3 +1,45 @@
-# Standard library (preview)
+# Standard Library (Preview)
 
-The standard library is located on the [Git repository](https://github.com/nushell/nushell/tree/main/crates/nu-std). At the moment it is an alpha development stage. You can find more documentation there.
+Nushell ships with a standard library of useful commands written in native Nu. By default, the standard library is loaded into memory (but not automatically imported) when Nushell starts.
+
+The standard library currently includes:
+
+- Assertions
+- An alternative `help` system with support for completions.
+- Additional JSON variant formats
+- XML Access
+- Logging
+- And more
+
+To see a complete list of the commands available in the standard library, run the following:
+
+```nu
+use std
+scope commands
+| where name =~ '^std '
+| select name usage extra_usage
+| wrap "std-lib"
+```
+
+In addition, `stdlib-candidate`, found in the [nu_scripts Repository](https://github.com/nushell/nu_scripts/tree/main/stdlib-candidate/std-rfc), serves as a staging ground for new commands before they are added to the standard library.
+
+::: tip Note
+Currently, parsing of the standard library impacts Nushell startup time. New commands are not being added to the library until this is resolved.
+:::
+
+To disable the standard library, you can start using:
+
+```nu
+nu --no-std-lib
+```
+
+You will not be able to import the library using `use std *`, nor use any of its commands, if it is disabled in this way.
+
+::: tip Did You Know?
+Because the standard library is simply composed of [custom commands](./custom_commands.html) in [modules](./modules.html) and [submodules](./modules.html#submodules-and-subcommands), you can see the source for each command with the [`view source`](/commands/docs/view_source.md) command. For example, to view the source for the `ellie` command (with syntax highlighting):
+
+```nu
+use std *
+view source ellie | nu-highlight
+:::
+```