Update gallery.yml (#1100)

Co-authored-by: Brigitta Sipőcz <bsipocz@gmail.com>

Author: Kisleif

docs/gallery.yml

@@ -1,3 +1,7 @@
+- name: Metrology Lecture Book
+  website: https://kisleif.github.io/mtbook/intro.html
+  repository: https://github.com/Kisleif/mtbook
+  image: https://kisleif.github.io/mtbook/_static/i_mt.png
 - name: Introduction to Python Programming
   website: https://vdsukhov.github.io/python-intro-course/intro.html
   repository: https://github.com/vdsukhov/python-intro-course