docs: fully document the bring-your-own-Bible corpus format

Expand "Bring your own Bible" with the details a user needs to actually do it:
- the six-column TSV format with three real, tab-marked example rows and a
  column legend;
- which columns are load-bearing -- clectio matches on Book+Chapter+Verse
  (cols 1,4,5) and prints col 6; Abbrev/BookNum (cols 2,3) are ignored but must
  be present;
- the Vulgate-versification requirement made explicit (a differently numbered
  Bible misaligns), with the "start from vul.tsv, swap the text column" tip;
- pointer to the four ready corpora shipped with lectio, with a runnable drb
  example;
- a "switching back to the Vulgate" note (the git-tracked text_*.lz blob is
  overwritten by a CORPUS= build).

Docs only; no code change.

Claude-Session: https://claude.ai/code/session_01S1CD1u3tgSS4xNu6WHfp5a

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