feat: add "Using the playground" section + Reference skeleton

Restructure the docs into five top-level sections, splitting the
application you drive from the database language you build with.

- New "Using the playground" section: command-line options; the assistive
  editor (completion, highlighting, [ERR]/[WRN] indicator, hints, in-line
  editing); the output pane (scrolling); projects (save/load/new/rebuild);
  undo/redo & history; export & import; clipboard; getting help. Grounded in
  the in-app help/usage and ADR-0003/0022/0027.
- Reference: seed the remaining topic pages (Columns, Relationships,
  Indexes, Constraints, Inserting & editing data, Querying & inspecting)
  with real syntax synopses; worked examples to follow.
- Surface the assistive editor on the landing page and in Getting started;
  restore cross-links now that targets exist.

Plan + STYLE updated to the five-section structure. 24 pages, build green,
links resolve, content clean; planned features carry "planned" callouts.

website/src/content/docs/getting-started/first-project.md

@@ -11,7 +11,9 @@ playground starts.
 
 When you launch `rdbms-playground` with no arguments, it opens a fresh
 temporary project for you. Type commands into the input field at the bottom
-and press <kbd>Enter</kbd> to run them.
+and press <kbd>Enter</kbd> to run them. As you type, the field completes
+commands with <kbd>Tab</kbd> and flags mistakes before you run — see
+[The assistive editor](/using-the-playground/the-assistive-editor/).
 
 ## Create a table
 
@@ -57,4 +59,5 @@ That is the whole loop: **create → add columns → insert → show**. From her
 
 - Build the full example database in [Build the library](/guides/build-the-library/).
 - Learn how [simple and advanced modes](/getting-started/modes/) differ.
+- Let the editor help you: [completion, highlighting, and live error flags](/using-the-playground/the-assistive-editor/).
 - Look up any command in the [Reference](/reference/types/).