DOC: docs: Document how to build the documentation

doc_developer/building_the_docs.md

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+# Building the docs
+
+This guide goes through how to build the NumPy-Financial documentation with poetry
+
+## Assumptions
+
+This guide assumes that you have set up poetry and a virtual environment. If you have 
+not done this please read [building_with_poetry](building_with_poetry.md).
+
+You can check that poetry is installed by running:
+
+```shell
+poetry -V
+```
+
+## Installing the documentation dependencies
+
+NumPy-Financial is built using [sphinx](https://www.sphinx-doc.org/en/master/) with [numpydoc](https://numpydoc.readthedocs.io/en/latest/).
+
+The dependencies can be installed using poetry and the ``docs`` group:
+
+```shell
+poetry install --with docs
+```
+
+## Building the documentation
+
+The documentation is located in the ``doc``  directory. The first step is to change directory into this directory
+
+```shell
+cd doc
+```
+
+Once you are in the ``doc`` directory, the documentation can be built using ``make``.
+
+```shell
+poetry run make html
+```
+
+This will create the docs as a html document in the ``build`` directory. Note that there are several options available,
+however, only the html documentation is built officially.