DOCX: a link's destination is dropped, label and all

[DemchaAV]

InlineTextRun.linkTarget() and a node-level DocumentLinkTarget both reach
DocxSemanticBackend and are dropped: the label survives as ordinary text, the destination
does not. #447 named this and deliberately left it out — "hyperlink relationships are a
larger piece of work (an XWPFHyperlinkRun plus an OPC relationship per link) and are better
tracked separately". This is that separate track.

Word expresses both halves, so this is not a format limitation:

• an external link is an XWPFHyperlinkRun plus a relationship in the part's .rels;
• an internal one is w:hyperlink w:anchor against a bookmark, which also needs
  DocumentBookmarkOptions to start emitting w:bookmarkStart/w:bookmarkEnd — today it is
  dropped too, so an anchor has nothing to point at.

The run walk that would carry it already exists: writeParagraphRuns reads each run's style
and has its linkTarget in hand at the same point.

Also update docs/architecture/backend-capability-matrix.md (the paragraph row still
names linkTarget as the reason it is ⚠️) and render-docx/README.md, which lists hyperlinks
and bookmarks under "not implemented".