Address review comments on create_full_archive.py

- Replace blind regex HTML surgery with BeautifulSoup for safe, structured
  href/src rewriting (addresses reviewer concern about brittle regex manipulation)
- Add _make_session() with urllib3 Retry for robust external downloads with
  backoff on 429/5xx responses (addresses reviewer concern about flaky downloads)
- Fix copy_tree_ignore_broken_symlinks() to copy resolved symlink targets
  instead of recreating symlinks, making archives portable across platforms
- Add _rewrite_url() helper to centralize /docs/ to relative path conversion,
  including /docs/ to ../index.html for Docs Home links
- Rewrite add_archived_banner() with BeautifulSoup instead of string replace
- Patch renderSidebar baseUrl: "/docs" to "" and replace href computation with
  archive-root-aware function using known subdirs detection so sidebar links
  work in file:// context regardless of where the archive is placed on disk

Author: ebembi-crdb