docs: pip3 install -r docs/requirements.txt as well

[agilgur5]

What does this PR do?

add pip3 install -r docs/requirements.txt to contributing directions

• this was missing, so would get an error:

  ❯ invoke docs.build-docs
  /bin/bash: mkdocs: command not found

  • no more error once docs deps are installed

Found this issue while contributing my first docs PR #396

What gif best describes this PR or how it makes you feel?

Source

Completion checklist

• Additions and changes have unit tests (N/A)
• Unit tests, Pylint, security testing, and Integration tests are passing.. GitHub actions does this automatically
• The pull request has been appropriately labeled using the provided PR labels