docs: add MRVPP ATBD v2 document

Add the Medium Resolution Vegetation Phenology and Productivity (MRVPP)
Algorithm Theoretical Basis Document, converted from PDF via pdf2md and
manually repaired to render cleanly:

- Rebuilt the BOX 1 / BOX 2 code blocks (a GDAL mosaic script and TOC had
  leaked into the BOX 1 fence; the script was split in two).
- Removed ~20 leaked PDF page-footer artifacts and a duplicated
  top-of-document block, and dropped trailing duplicate content.
- Reconstructed Figure 10 from its three real panels (SOSD, EOSD+AMPL,
  TPROD), grouped in a layout div with the caption bound to the figure.
- Included only the 14 images the document references.

Verified: validate_qmd_files.py passes and quarto render produces HTML
and PDF without errors.

Author: MatMatt