docs(site): switch to Roboto Flex and align light-theme prose colors

After scraping docs.docker.com's stylesheet, two of our font/color
choices turned out to be subtly off the mark:

1. The site\u2019s --font-sans is \"roboto flex\", not \"Roboto\".
   Roboto Flex is the variable-axis superset Docker self-hosts at
   /assets/fonts/RobotoFlex.woff2; on this site we pull it from
   Google Fonts (which does serve it as opsz 8..144 / wght 100..900).
   Mono stays as Roboto Mono \u2014 docs.docker.com uses the same file,
   they just alias it as \"roboto flex mono\" internally.

2. Light-theme body copy was rendered in --gray-900 (\"#1E2129\") \u2014
   essentially black. docs.docker.com\u2019s prose body is
   oklch(37.3% .034 259.733) \u2248 \"#3F4750\" (cool mid-grey), with
   headings at oklch(21% .034 264.665) \u2248 \"#1F242D\". Lift those
   two values into the light-theme --text and --text-secondary
   tokens. Dark-theme tokens already lined up with their inverse
   prose values (\"#C8CFDA\" \u2248 oklch(87.2%)) so no change there.

Heading weights (h1: 800, h2: 700, h3: 600) and the gray scale were
already aligned in earlier commits; this just brings the remaining
type-system bits in sync.

Author: dgageot

docs/_layouts/default.html

@@ -16,7 +16,7 @@
 
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-  <link href="https://fonts.googleapis.com/css2?family=Roboto:wght@400;500;700;900&family=Roboto+Mono:wght@400;500&display=swap" rel="stylesheet">
+  <link href="https://fonts.googleapis.com/css2?family=Roboto+Flex:opsz,wght@8..144,100..900&family=Roboto+Mono:wght@400;500&display=swap" rel="stylesheet">
   <link rel="stylesheet" href="{{ '/css/style.css' | relative_url }}">
   <link rel="icon" type="image/svg+xml" href="{{ '/assets/docker-mark.svg' | relative_url }}">
 </head>

docs/css/style.css

@@ -94,11 +94,13 @@
   --header-height: 56px;
   --toc-width: 220px;
 
-  --font-sans: 'Roboto', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
-  --font-mono: 'Roboto Mono', 'SF Mono', Menlo, 'Cascadia Code', monospace;
+  --font-sans: 'Roboto Flex', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  --font-mono: 'Roboto Mono', ui-monospace, 'SF Mono', Menlo, Monaco, Consolas, monospace;
 }
 
-/* Light theme override — calibrated to docs.docker.com */
+/* Light theme override — calibrated to docs.docker.com.
+ * Light body text is intentionally a mid-grey (\"#3F4750\"), not pure
+ * black, to match the prose style on docs.docker.com. */
 [data-theme="light"] {
   --bg: var(--bg-light);
   --bg-sidebar: #F3F4F6;
@@ -107,8 +109,8 @@
   --bg-card: #FFFFFF;
   --bg-hover: var(--gray-100);
 
-  --text: var(--gray-900);
-  --text-secondary: var(--gray-700);
+  --text: #1F242D;                /* headings / strong copy */
+  --text-secondary: #3F4750;      /* body — docs.docker.com prose body */
   --text-muted: var(--gray-500);
   --text-sidebar: var(--gray-700);
   --text-sidebar-active: var(--gray-900);