Move docs into submodule (fixes #1342) (#1343)

.gitmodules

docker/dev/Dockerfile

docker/dev/volume_mount.dockerfile

docker/prod/Dockerfile

docs

docs/.gitignore

docs/book.toml

docs/img/chat_screen.png

docs/img/main_screen.png

docs/img/rank_algorithm.png

docs/src/SUMMARY.md

docs/src/about/about.md

docs/src/about/features.md

docs/src/about/goals.md

docs/src/about/guide.md

docs/src/about/ranking.md

docs/src/administration/administration.md

docs/src/administration/backup_and_restore.md

docs/src/administration/configuration.md

docs/src/administration/install_ansible.md