Give every chapter page a level-0 title

[Bill Traynor (wmat)]

The Antora site titles 17 of its 18 pages "Untitled" — in the browser tab and
anywhere else the UI prints a page title. This gives them real titles, and updates
the guide so template users don't hit the same problem.

Cause

Antora takes a page's title from its level-0 = Title line. These pages opened with
a level-1 == heading instead. The live site proves it: install-tools.adoc was the
one page with = Install Asciidoctor..., and the only one with a real title.

Not a regression. The chapters used == before the migration too, because they were
written as sections of one master document. MIGRATION.md Step 7 requires the level-0
title; this repo never met it, and nothing checks for it.

What changes

Pages. Every heading moves up one level (== → =, === → ==, …), so each
page opens with one = Title and its sections keep their relative depth. Lines inside
listing, literal and fenced blocks are untouched — asserted per file, which matters for
a_few_basics.adoc, whose fenced example of heading syntax contains = Title head.

Assembler. Every include gains leveloffset=+1, turning each page's level-0 title
back into a chapter. The [preface] and [bibliography] style lines move from the top
of copyright.adoc and bibliography.adoc into the assembler, ahead of the include, so
each page starts with a bare title. The template already does this for contributors.

Guide text. The "When adding a new chapter" steps in antora-structure.adoc and the
chapter-file paragraph in author-quickstart.adoc now say to start each chapter with a
level-0 title and include it with leveloffset=+1. Neither was mentioned before, so an
author following this guide got a green build and an "Untitled" page. Also fixes an
include: → include:: typo.

Please review: vale.adoc changes the PDF

vale.adoc had three peer == sections, and a page can have only one level-0 title. Only
its first heading was promoted, so "AsciiDoc style for Vale" and "RISC-V style for Vale"
now sit under "Vale at RISC-V".

• Site: unchanged apart from the page gaining its title.
• PDF: those two stop being chapters. 15 chapters become 13, chapters 11–15
  renumber to 9–13, and the PDF is one page shorter (75 → 74).

The alternative is splitting vale.adoc into three pages.

Deliberately not changed: index.adoc

It's the cover, with the logo above the heading. The shared UI renders an <h1> from the
page title unconditionally — partials/article.hbs has no notitle escape — so titling it
puts the heading above the logo. That's a cover-layout decision for docs-spec-template,
whose index.adoc has the same problem — raised upstream as riscv/docs-spec-template#155, along with a CI check that would catch pages like these.

Verification

Compared against a baseline build of main:

Check	Before	After
Untitled pages (Antora)	17 of 18	1 of 18 (index.html)
Antora log	1 warn, 0 errors	1 warn, 0 errors (same pre-existing graphviz warning)
Asciidoctor warnings (PDF/HTML)	0	0
PDF	15 chapters, 75 pages	13 chapters, 74 pages

• Rendered body text of the standalone HTML differs only in the new guide sentences —
  so shifting the headings and adding leveloffset=+1 reproduces the document exactly.
• Heading outline differs only in the Vale subtree.
• Front matter, TOC, lists and index all present in the PDF.

Follow-up: nav order

A commit putting the title page, copyright and contributors first in the nav arrived on
this branch after this PR merged, so it isn't part of it. It's #172.

Relationship to #170

Independent — no files in common. The home/index.html 404 on the header logo and
breadcrumb is still present on this branch; #170 fixes that.

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