I am reading the paper Chern-Weil map for principal bundles over groupoids. In page number $13$, authors say

let us recall the definition of fundamental group of a topological groupoid.

But, they do not give any reference (or I could not see where they have given reference) for original paper, where the notion of fundamental group of topological/Lie groupoid is introduced.

Can some one point me to some reference where this has been mentioned for the first time?

Side question : Do we have notion of fundamental group for ``other kinds" of algebraic/differential geometrical objects? Do we have fundamental group of, say, an algebraic variety, or a scheme or similar such thing?

There is another way to introduce the notion of fundamental group of Lie groupoid (Page $187$ in LMS Lecture notes series, titled Poisson geometry, deformation quantisation and group representations, Simone Hurt, John Rawnsley and Daniel Strenheimer (eds)). But, that is different from explanation in the above mentioned paper.