Improve documentation page and overlay layout

Make the embedded and non-embedded documentation more readable:

* Constrain the prose to a ~44em measure with a calm vertical rhythm
  (left-aligned, unitless line-height, consistent paragraph/list
  spacing), scoped to the doc pages (main.tutorial), so lines are no
  longer viewport-wide on large screens.
* Cap the documentation overlay window (#tutorial) at 72em instead of
  spanning ~96% of the viewport, and anchor it to the right so it reads
  as a right-hand documentation drawer that leaves the main workspace
  (query / results) uncovered. Small viewports still get a full-screen
  panel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: Iabdfdc8b03c3713abce89b7ecdea6513190a3cfd
diff --git a/dev/scss/main/tutorial.scss b/dev/scss/main/tutorial.scss
index 44654dd..9fa882f 100644
--- a/dev/scss/main/tutorial.scss
+++ b/dev/scss/main/tutorial.scss
@@ -14,8 +14,13 @@
   z-index:          9999;
   top:              5%;
   bottom:           5%;
-  left:             2%;
+  // Anchor the panel to the right and cap its width, so it reads as a
+  // right-hand documentation drawer and leaves the main workspace on the left
+  // uncovered. On narrow viewports it still uses (almost) the full width (see
+  // the media query, which resets left/right/width for a full-screen panel).
+  left:             auto;
   right:            2%;
+  width:            min(96%, 72em);
   padding-right:    lengths.$right-view-distance;
   background-color: colors.$nearly-white;
   box-shadow:       choose.$choose-box-shadow;