Make the Jekyll docs usable on mobile

[jowens]

Fixes #47.

On a phone the article text was rendered entirely off-screen. What you saw in @greggman's video was the sidebar, and only the sidebar.

Root cause: two bugs that compound

1. .docs-container never became a column on mobile. It's display: flex (style.scss:188). The 768px block set .docs-sidebar to width: 100%; position: static but never set flex-direction: column. The sidebar keeps flex-shrink: 0, so it took the full 390px and the content column was laid out beside it, starting at x=410.

2. .docs-content { max-width: calc(100vw - 280px) } was never overridden. That 280px is the desktop sidebar width. Neither media query touched max-width. On a 390px viewport it computes to exactly 110px.

body { overflow-x: hidden } then clipped the off-screen content with no scrollbar, so there was no way to reach it.

Measured, headless Chrome at 390x844

	before	after
.docs-content width	110px	390px
.docs-content position	x = 410 → 480 (off-screen)	x = 0
paragraph text width	70px	358px
page horizontal overflow	222px (clipped)	0px
reachable nav links	4	25

Desktop at 1440 is unchanged: content 1160px at x=280, sidebar fixed, no hamburger.

Also in this PR

• Mobile hamburger menu. The markup already existed in header.html but .menu-icon and .nav-trigger were display: none with nothing re-enabling them at any width, and the real nav opened only on :hover, which touch devices lack. The mobile nav rules have to live after the .nav-dropdown block — they override display: none at equal specificity, so source order decides. That ordering was the difference between 4 and 25 reachable links.
• Sidebar reordered below the article on mobile, so doc pages don't open with 15+ nav links before the first word of content.
• Pinch-zoom restored — dropped maximum-scale=1, user-scalable=no, adopting the convention @greggman landed for the examples in Some Tweaks for Mobile #46.
• Wide tables get their own horizontal scroll box; images constrained.
• min-width: 0 on .docs-content so a long unbreakable line in a <pre> can't widen the column past its container (flex items default to min-width: auto).
• A 480px breakpoint for phone-sized type and padding.
• Removed body{zoom:1}, body{overflow-x:hidden}, .site-nav{float:right} (a no-op under flex), and the unused .site-footer rules.
• .doc-btn and nav links bumped to >=44px tap targets.

Not included

docs/index.html and docs/index_files/ (692K of vendored Quarto/Bootstrap output) were deleted separately at @jowens' request — orphaned build output with no .qmd source and nothing referencing it. They were untracked, so they don't appear in this diff. docs/index.md is the real homepage and is untouched.

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