Move documentation related files into subdirectory to clean up root dir of project

docs/source/assets/images/geant_ip_ports_diagram.png

docs/source/assets/images/geant_logo_white.png

docs/source/assets/images/geant_logo_white.svg

docs/source/assets/images/image_sources/Architecture.drawio

...ce/assets/images/image_sources/Legacy_GAP_diagrams.drawio

docs/source/assets/images/image_sources/tnc_diags.drawio

docs/source/assets/images/logo.png

docs/source/assets/stylesheets/extra.css

docs/source/index.md

docs/test-docs.sh

docs/vale/.vale.ini

docs/vale/styles/config/vocabularies/geant-jargon/accept.txt

docs/vale/styles/custom/Contractions.yml