Deprecate date to-record and date to-table (#14319)

# Description

Implements #11234 based on the comments there:

* (Previously implemented): `into record` handles nanoseconds (as well
as milliseconds and microseconds, which the deprecated commands didn't
support).
* Added deprecation warning to `date to-record` and `date to-table`
* Added new example for `into record` showing the conversion to a table
* Changed `std/dt` to use `into record`
* Added "Deprecated" category back to nu-protocol::Signature
* Assigned the deprecated commands to the Deprecated category so be
categorized properly in the online Doc.

# User-Facing Changes

Deprecated command warning

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

Searched doc for existing uses of `date to-record` and `date to-table`:

* For primary English-language docs, there are no uses other than in the
auto-generated command help, which will be updated based on this PR
* Other language translations appear to have an old use in several
places and will need to be updated to match the English-language doc.

crates/nu-protocol/src/signature.rs

@@ -44,6 +44,7 @@ pub enum Category {
     Date,
     Debug,
     Default,
+    Deprecated,
     Removed,
     Env,
     Experimental,
@@ -79,6 +80,7 @@ impl std::fmt::Display for Category {
             Category::Date => "date",
             Category::Debug => "debug",
             Category::Default => "default",
+            Category::Deprecated => "deprecated",
             Category::Removed => "removed",
             Category::Env => "env",
             Category::Experimental => "experimental",