Migrate Boost.Graph documentation to Antora

NOTE: There is no way to split this PR properly, as all .html files need to be replaced all at once.
NOTE: The ~40000 lines of change in doc source code don't need direct review. Instead please review the preview currently hosted on the fork's Github Pages: https://becheler.github.io/graph/graph/index.html

This PR switches Boost.Graph from the pre-baked BoostBook/Quickbook HTML at doc/*.html to an Antora source tree under doc/modules/ROOT/. Once merged, the next Boost release will publish the new site at www.boost.org/doc/libs/<version>/libs/graph/ automatically (release-tools auto-detects doc/build_antora.sh).

Changes

• New Antora source tree at doc/modules/ROOT/ with 209 .adoc pages and 154 paired .cpp/.txt example sidecars.
• Wires release-tools' Antora pipeline via doc/build_antora.sh, doc/package.json (antora 3.1.14 + lunr extension), and doc/package-lock.json for reproducible installs.
• Deletes 180 legacy HTML files at doc/*.html / doc/*.htm, superseded by the Antora pages.
• Deletes legacy image assets doc/figs/ (106 files) and doc/stoer_wagner_imgs/ (15 files); the new doc carries its own copies under doc/modules/ROOT/assets/images/.
• Deletes 3 partial reST remnants (read_graphml.rst, read_graphviz.rst, write_graphml.rst), replaced by doc/modules/ROOT/pages/io/{graphml,graphviz}.adoc.
• Deletes doc/Makefile (LaTeX/pdflatex/dvips rules from 2002, no longer used by any toolchain in the repo).

Verification

• cd doc && ./build_antora.sh playbook.yml succeeds end-to-end and produces doc/build/site/graph/index.html.

Known follow-ups (not blocking this PR)

• I had to delete the develop louvain html documentation on this PR to solve conflicts: it will require a new .adoc documentation once this PR ships.
• CI freshness checks for examples output (git diff --exit-code on .txt after rebuild), may be best to defer until the random-generator examples are RNG-seeded so the diff is deterministic. Also maybe over-engineered.