docs: Use flexbox (for Hugo documentation)

[MarkDBlackwell]

Fixes #2373 #2938 #2955 #2956 #2957 #2958 #2959 #2960 #2961 #2962 #2963 #2964 #2965 #2966 #2967 #2968 #2969.
(See also #1725 #1523 #1362.)

Provides a flexbox layout for Hugo's documentation (based on this model) which is visually identical to the current one, except for the following features:

• Space efficiency: more document content immediately visible (on desktop):
  • Header height lessened
  • Next/previous navigation arrows:
    • Extra horizontal padding removed
    • Stacked (vertically, conserving horizontal space)
    • Use no width if absent
  • Multi-document menu:
    • Adjusts to widest entry
    • Sub-entry groups fully visible (vertically)
    • No extra horizontal padding when toggled to hidden
• Responsive on smartphones:
  • Multi-document menu:
    • No longer disappears on resizing
    • Arrows:
      • Better positioning (vertically)
      • Corrects "staircase" bug (seen in Chrome & Opera)
  • Tested with developer tools
• Printing
• Functional with or without JavaScript — now, these fall back gracefully:
  • Hamburger (for smartphones)
  • Menu expansion
• Multi-document menu:
  • Top level, single entries (which lack sub-entries) now highlight, when selected
  • Sub-entries:
    • Right-justified (on desktop and smartphones)
    • Centered (on wide smartphones)
  • Scrollbar position remembered while toggling
  • No longer partially remains, when toggled to hidden (sometimes)
• Table of Contents:
  • Scrolls (on overflow)
  • Text wraps
  • On narrow desktops:
    • No longer disappears
    • Partially transparent overlay
• GitHub button styling unified (visually), since all are (equally) clickable
• Tooltips added, for:
  • Hamburger
  • Logo (and version)
  • Multi-document menu entries (including sub-entries)
  • Each GitHub button
• Accessibility:
  • Keyboard users:
    • Focus indications added:
      • Hamburger
      • Version (and logo: both link to the main page)
  • Screen readers (and keyboard users):
    • Tab order improved (except Firefox, due to this bug):
      • Forward:
        • Document content is placed first in the DOM (after the search box)
      • Backward, from the end:
        • Header is placed nearest:
          • Hamburger is reached first
        • Next/previous navigation arrows are placed next (-nearest)
        • Multi-document menu is third (-nearest)
        • Table of Contents is fourth (-nearest)
• Site Showcase widened to 4 columns, except:
  • 3 columns, on narrow desktops
  • 1 column, on smartphones
  • 2 columns, on wide smartphones
• Includes Autoprefixer output for:
  • Last 2 versions (of all major browsers)
  • Browsers > 1% (selected by global usage statistics)
• Readable "view page source" (to help new users)

Tested on:

• Firefox
• Google Chrome
• Opera (latest for desktop, plus version 36 for Windows XP)
• IE 11 (plus 10 by emulation)

[bep]

I appreciate the effort, but we need to land the discussion on the docs before we can take PRs.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.