DOC: Low-priority: Add Glossary

[asoplata]

We should maybe add, expand, and interlink Glossary definitions throughout the textbook. The original Glossary page from the Developer-website is pasted below (and will be removed soon from the Developer-website):

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Glossary

The Glossary provides short definitions of HNN-specific vocabulary and
general computational neuroscience concepts. If you think a term is
missing, please consider creating a new issue or opening a pull request
to add it.

:sorted:

drive
    An exogenous source of excitatory action potentials that perturbs
    the cortical column network.

gid
    The "global" cell ID. A gid is assigned to each cell in the
    network as well as artificial drive cells. Global refers to the fact
    that the cell ID is unique even if the simulation is split across
    cores.

distal
    Anatomical location that is further away from the soma. Distal
    drives refer to extrinsic connections to the distal dendrites of the
    cortical column, which often originate from indirect thalamic
    nuclei, as well as other cortical regions.

proximal
    Anatomical location that is closer to the soma. Proximal drives
    refer to extrinsic connections to the proximal dendrites of the
    cortical column, which often originate from direct thalamic nuclei.