Add API overview diagram to user guide (#597)

Author: Hrovatin

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- API diagram in user guide
+
 ### Fixed
 - `Campaign` no longer allows overlapping names between parameters and targets
 

CONTRIBUTORS.md

@@ -27,7 +27,7 @@
 - Julian Streibel (Merck Life Science KGaA, Darmstadt, Germany):\
   Bernoulli multi-armed bandit and Thompson sampling
 - Karin Hrovatin (Merck KGaA, Darmstadt, Germany):\
-  `scikit-fingerprints` support
+  `scikit-fingerprints` support, API overview diagram
 - Fabian Liebig (Merck KGaA, Darmstadt, Germany):\
   Benchmarking structure and persistence capabilities for benchmarking results
 - Alexander Wieczorek (Swiss Federal Institute for Materials Science and Technology, Dübendorf, Switzerland):\

docs/_static/api_overview_dark.svg

@@ -1,5 +1,25 @@
 # User Guide
 
+The most commonly used interface BayBE provides is the central 
+[`Campaign`](baybe.campaign.Campaign) object,
+which suggests new measurements and administers the current state of 
+your experimental operation. The diagram below explains how the 
+[`Campaign`](baybe.campaign.Campaign) can be used to perform 
+the bayesian optimization loop, how it can be configured and 
+how the results can be post-analysed.
+
+```{image} ../_static/api_overview_dark.svg
+:align: center
+:class: only-dark
+```
+
+```{image} ../_static/api_overview_light.svg
+:align: center
+:class: only-light
+```
+
+Detailed examples of how to use individual API components can be found below:
+
 ```{toctree}
 Getting Recommendations <getting_recommendations>
 Campaigns <campaigns>