Replace cng-cookbook-contribution.qmd with cng-format-validation-sections.qmd

Three per-format validation sections designed to slot into the CNG Formats Guide:
- Validating Your FlatGeobuf (spatial index, CRS metadata, format decision table)
- Validating Your GeoParquet (geo metadata, CRS encoding, compression, bbox column, format decision)
- Validating Your Shapefile Before Migration (multi-file integrity, 2 GB limit)

Each section follows concept → why it matters → how to verify → how to fix.
Tissot appears only as a "See also" in the contributor note.
Deletes old Tissot-centric cookbook.

Co-authored-by: chrislyonsKY <135981795+chrislyonsKY@users.noreply.github.com>

Author: Copilot