feat: publish the archived release PDFs, and archive new ones automatically

[achubaty]

Draft: depends on PredictiveEcology/SpaDES.docs#16, which adds
publishManualArchive(). Also best merged after #20, which stops committing
docs/.

The four archived manuals (v1.0.0–v1.0.3, ~7 MB) are tracked under
archive/pdf/ and have never reached the site — archive/ sits at the project
root while the deploy publishes docs/.

change	effect
publishManualArchive() in build.R	copies the archive into docs/archive/pdf/ and writes an index page built from the files present
static link in the preface to archive/	never needs editing; the page behind it regenerates each build
version from DESCRIPTION	build.R hardcoded "1.0.4" with an "update this for each new release" comment — a second source of truth, and a step to forget
"Archive the release PDF" workflow step	commits a newly archived PDF back to main as the bot

The workflow step is self-gating: archiveManualPDF() writes
archive/pdf/LandR-manual-v<version>.pdf only when a PDF was rendered and
that version is not already archived. So it is a no-op on ordinary builds, needs
no separate release trigger, and cannot be skipped at release time.

Not yet exercised

No PDF is produced today — _output.yml declares pdf_book but build.R
renders bs4_book only, because xelatex + krantz.cls + biblatex are unproven
on modern R. So this PR publishes the four existing archives and leaves the
release path dormant until the PDF builds. Getting the PDF working is the
prerequisite for the release scheme, and is worth its own issue.

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