feat: simple-mode code-block highlighting, prompt, and copy rules

Add a custom Shiki grammar for the simple-mode command language
(src/grammars/rdbms.mjs), registered with Expressive Code. Two language ids
share it: rdbms (real commands) and rdbms-syntax (abstract templates).
Simple-mode blocks now highlight; advanced examples keep sql.

Separation + copy ergonomics via CSS (global.css): a decorative, copy-safe
"> " prompt on rdbms command lines (not in the copy buffer), and the copy
button hidden on multi-command rdbms blocks and on rdbms-syntax templates
(the app input is single-line, so a multi-command paste is not runnable);
single-command, sql, and sh blocks keep copy.

Content: convert 22 simple-mode fences to rdbms; lead the simplest examples
(first project, Tables reference) with bare "with pk" (the beginner default
that creates a ready-made id key), pointing to the named form. Record the
fence + prompt conventions in STYLE.md.

website/src/content/docs/reference/tables.md

@@ -11,16 +11,24 @@ in the Reference.
 
 ## Create a table (simple mode)
 
-Every table is created with its primary key. The `with pk` clause lists the
-primary-key column(s):
+The simplest way to create a table is `with pk` on its own — it gives the
+table a ready-made primary-key column named `id` that the database fills in
+for you:
 
-```text
+```rdbms
+create table authors with pk
+```
+
+Prefer to name the key, or give it a specific type? Name it in the `with pk`
+clause:
+
+```rdbms
 create table authors with pk author_id(serial)
 ```
 
-Other columns are added afterwards with `add column`:
+Either way, the rest of the columns are added afterwards with `add column`:
 
-```text
+```rdbms
 add column to authors: name (text)
 add column to authors: birth_year (int)
 ```
@@ -30,14 +38,14 @@ add column to authors: birth_year (int)
 To make the primary key span more than one column, list them, comma
 separated:
 
-```text
+```rdbms
 create table loans with pk book_id(int), member_id(int)
 ```
 
 **Syntax**
 
-```text
-create table <Name> with pk <col>(<type>)[, ...]
+```rdbms-syntax
+create table <Name> with pk [<col>(<type>)[, ...]]
 ```
 
 ## Create a table (advanced mode)
@@ -58,13 +66,13 @@ keys — see the Constraints and Relationships references.
 
 **Syntax**
 
-```text
+```rdbms-syntax
 create table [if not exists] <Name> (<col> <type> [constraints], ...)
 ```
 
 ## Drop a table
 
-```text
+```rdbms
 drop table authors
 ```