style(layout): lift prose max-width to fill content panel

- main p max-width: 88ch -> 105ch (~759px -> ~842px on Geist at 16px)

Effect on /docs/en/ch01-overview at 1440px:
- Numbered list items now each fit on a single line where they
  previously wrapped (1, 2, 3 of 'three reasons')
- Prose lines stretch from ~750px to ~845px, matching the content
  panel width so there's no dead space on the right
- Build clean: 35 pages in 2.53s

Author: hypnguyen1209

src/styles/custom.css

@@ -169,7 +169,7 @@ main li {
 }
 
 main p {
-  max-width: 88ch;
+  max-width: 105ch;
 }
 
 /* Tabular numbers for stats / data */