Remove duplicated dependencies list

[OrkoHunter]

We have listed Python dependencies in two places for this project

1. https://github.com/backstage/mkdocs-techdocs-core/blob/main/requirements.txt
2. install_requires in https://github.com/backstage/mkdocs-techdocs-core/blob/main/setup.py

This can cause inconsistencies when upgrading versions (like #18). So we should keep just one - possibly reuse data from requirements.txt in install_requires of setup.py.

[iamansoni]

I would like to work on this task; you can assign it to me.
Also, will it be a good solution if I make requirements.txt to be read as a file in the setup.py and pass it as a list of strings to install_requires ?

[OrkoHunter]

Sounds good to me. @iamansoni