💄🔧：keep the docs index styles off the docs pages

[DerekNonGeneric]

docs.liquid gives every page under /docs/ the body class docs, which is
the class the index's own styles read. So the index's resets applied to all
thirteen pages beneath it:

.docs main h1, .docs main h2, .docs main p,
.docs main dl, .docs main dd, .docs main ul { margin: 0 }
.docs main ul { padding: 0; list-style: none }

That is 123 paragraphs, 31 headings and 14 lists rendering with no spacing,
and lists with neither bullets nor indent. The contributing guide, the
commit-message page and the command-line syntax page are the worst hit, being
the longest.

Scoping to main was meant to keep those resets off the navbar and footer,
and it does. It gives no protection against another layout sharing the body
class, because that layout has a main too.

The fix

The index carries docs-index alongside docs, and its stylesheet reads the
new one. docs goes back to meaning only what the navigation uses it for, so
the Docs item still highlights on the index and on every page under it.

Checks

nps test passes, all 17. No rule matches .docs main any more. The index
renders identically: 307 words, 9 handbook rules, 6 marked unwritten, "3 of 9
written", 4 community docs. Nav highlighting confirmed on the index, on a doc
page, and on the home page.

Not in scope

The pages themselves are still on the old shell: deep-page.liquid puts them
in a 48ch column wrapped in .grid, .unit, .four-fifths and .improve,
none of which are styled anywhere. That is a redesign, and it is next.

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