DOC: Convert developer docs from Markdown to restructured text

[Kai-Striega]

Issue with current documentation:

The two files for developer docs:

• building_with_poetry.md
• getting_the_code.md

are currently markdown files. Although Sphinx supports Markdown it does not support many of the same features as restructured text.

Idea or request for content:

To enable better support for the documentation, these files should be converted to restructured text

[Kai-Striega]

An alternative to this would be to investigate how markdown files interact with sphinx, however I would prefer to have the same file types for consistency.