Add a rail-less landing layout to the docs template

What does this MR do and why?

I want a site's index page to use the full width, because the rail and its gutter leave a landing 1048px at 1440. Layout takes landing (a markdown page takes landing: true in its frontmatter) and renders without the rail, and one rule in wide.css sets the shell to a single column, so the content spans the 1400px .page-wide already sets and lines up with the header. The top nav still lists every section, so nothing the rail linked becomes unreachable, and a page that leaves the option off renders as before.

References

None.

Standards affected

None.

Exemption implications

None.

Test plan

• just guard, just validate, just check, just build and just agent pass.
• Rail on the landing: absent at 390, 721, 1010 and 1440. Rail on /guides/: present at all four.
• Landing content width, with the rail's width on /guides/ beside it: 357 (357) at 390, 671 (671) at 721, 945 (665) at 1010, 1328 (1048) at 1440. The header bar is 1400 at 1440, so the landing's content edges match it.
• No horizontal page scroll at 390, 721, 1010 or 1440, on either page.
• Phone: the landing's column is the same width as before the rail went; the phone loses the closed "On this page" disclosure above the content.
• A markdown page with landing: true builds without the rail.
• Not tested: Safari, Firefox, a screen reader.
• Known limit: the template links chrome v2.6.0, which has no .page-landing rule, so an opted-in page renders its content in the 240px column until the release that repins the template; a page that does not opt in is unaffected.
• Known limit: a landing page has no "On this page" group, since the rail carried it.

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