Initial commit

Co-authored-by: Aradhana Dube <a.dube@rug.nl>
Co-authored-by: Renzo I. Barraza Altamirano <r.i.barraza.altamirano@rug.nl>
Co-authored-by: Paolo Gibertini <p.gibertini@rug.nl>
Co-authored-by: Luca D. Fehlings <l.d.fehlings@rug.nl>

docs-site/docs/neuron_models/fhn/README.md

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+# Circuit implementing the fhn neuron.
+
+- The circuits in the schematics implement the FHN neuron described.
+- The FHN neuron is an implementation of the circuit described in (Ribar, L. (2019). Synthesis of neuromorphic circuits with neuromodulatory properties [Apollo - University of Cambridge Repository]. https://doi.org/10.17863/CAM.53750). The OTA and CMFB are well known designs that can be found in textbooks.

docs-site/docs/neuron_models/fhn/index.md

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+# FitzHugh-Nagumo
+
+## Circuit equation
+
+$$
+\begin{align}
+    C\frac{dv}{dt} &= I_{app} - I_{passive} - I_{fast} - I_{slow} \\
+    \frac{dv_{slow}}{dt} &= \frac{v - v_{slow}}{\tau_{slow}} \\
+    \frac{dI_{app}}{dt} &= -\frac{I_{app}}{\tau_{syn}}
+\end{align}
+$$
+
+where the currents are:
+- $I_{passive} = g_{max}(v - E_{rev})$
+- $I_{fast} = a_{fast} \tanh(v - v_{off,fast})$
+- $I_{slow} = a_{slow} \tanh(v_{slow} - v_{off,slow})$
+
+## Examples
+
+See the following interactive notebook for a practical example:
+
+- [Basic Usage Example](fhn.ipynb) - Introduction to the FitzHugh-Nagumo model