Put the title page and front matter first in the nav

[Bill Traynor (wmat)]

Follow-up to #171. That PR merged before this commit arrived, so it needs its own.

Puts the nav in the same order as the template and the PDF: title page, copyright,
contributors, introduction, then the chapters.

What changes

nav.adoc now follows the template's structure: the title page (index.adoc) is the
top-level entry, and copyright, contributors, the introduction and the chapters are
nested under it, in that order. The PDF already opens title page → copyright →
contributors → Chapter 1.

• The nav had no title-page entry at all. MIGRATION.md Step 9 asks for one
  ("landing page first").
• Copyright and contributors were last. Restore the template's front and back matter #169 put them there on the stated grounds
  that the central playbook numbers this component's chapters by line number in this
  file. That was never checked, and it's wrong: docs-dev-guide's content source is
  commented out in both antora.riscv.org and antora-dev.riscv.org, and neither has a
  numbering rule for docs-guide. The comment claiming otherwise is replaced by the
  template's own header comment, which warns about line-number coupling in general terms
  — true once this guide is registered centrally.
• The .Authoring and Editing RISC-V Specs nav title is dropped. The title-page entry,
  labelled with the component title, takes its place, as in the template.
• The guide taught the wrong nav. The example in antora-structure.adoc — the one
  template users copy — listed spec-sample.adoc as the landing page (the template's is
  index.adoc), put contributors after the chapters, and left out copyright. It now
  matches the template's nav.adoc entry for entry.

Verification

• Same page set as before, plus the title page.
• Antora log unchanged: one pre-existing graphviz warning, no errors.
• Rendered sidebar: RISC-V Authoring and Editing Guide › Copyright and license
  information › Contributors › Introduction › …, nested under the title page.

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