Bump richdoc to v0.2.0 and wire its three new inline nodes into the ODT
converter in both directions, moving them off the Raw fallback:

- Footnote  <-> text:note (footnote/endnote; note-body block content;
  citation/id regenerated on write; endnote normalises to footnote).
- Anchor    <-> text:bookmark (also parses text:bookmark-start; consumes
  a stray text:bookmark-end). Point marker; any Inlines written adjacent.
- CrossRef  <-> text:bookmark-ref (also parses text:reference-ref). RefCite
  has no ODF element, so it is a best-effort bookmark-ref tagged
  odfgo:cite="true" that Parse restores to RefCite.

Adds round-trip and error-branch tests (total line coverage 100.0%) and
documents the mappings, the endnote->footnote and RefCite best-effort
choices in doc.go and the README.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>