[GRASS-git] [OSGeo/grass-website] 67e8b3: Add Tangible Modeling... book 2nd edition (#154)

[Vaclav Petras]

  Branch: refs/heads/master
  Home:   https://github.com/OSGeo/grass-website
  Commit: 67e8b3c12fb5f9682c22906a6bcf8b97155f47ff
      https://github.com/OSGeo/grass-website/commit/67e8b3c12fb5f9682c22906a6bcf8b97155f47ff
  Author: Vaclav Petras <wenzeslaus at gmail.com>
  Date:   2020-05-21 (Thu, 21 May 2020)

  Changed paths:
    M content/learn/books.md
    M static/images/gallery/books/book_petrasova_2015_tangible.jpg
    A static/images/gallery/books/book_petrasova_2018_tangible.jpg

  Log Message:
  -----------
  Add Tangible Modeling... book 2nd edition (#154)

Additionally, use DOI as links and update images.