Add automatic documentation build and deploy for master and develop branches

Actions required on @lambadalambda 's part before this can be merged:

1. Install rsync on the target server
2. Create a new CI user on the target server and get an ed25519 ssh key for it (ssh-keygen -t ed25519)
3. Copy the private key and paste it to CI/CD environment variables with SSH_PRIVATE_KEY as a key, mark it protected
4. Copy the entry for the server from known_hosts and paste it to CI/CD environment variables with SSH_HOST_KEY as a key, mark it protected
5. Create a static directory and give the CI user write permissions to it
6. Enter the rsync destination string (e.g gitlabci@pleroma.social:/var/www/pleroma-docs) to CI/CD environment variables with SSH_USER_HOST_LOCATION as a key, mark it protected
7. Set up a static server to serve content from $static_dir/master on docs.pleroma.social and content from $static_dir/develop on docs-develop.pleroma.social (or whatever you would like to call it)
8. If you have a non-standart ssh post enter it in CI/CD environment variables with SSH_PORT as a key, mark it protected

closes #478 (closed)