docs: add MRVPP ATBD v2 and PUM v1

[MatMatt]

Adding the two MR-VPP documents to the public library:

• MRVPP ATBD v2 — Algorithm Theoretical Basis Document for Methane Retrieval – Vegetation Photosynthetic Potential
• MRVPP PUM v1 — Product User Manual for MR-VPP

Both documents have been reviewed and tested on develop (PRs #58, #61–#65).