Refactor Mattermost slash commands docs

doc/project_services/img/mattermost_add_slash_command.png

doc/project_services/img/mattermost_bot_auth.png

...roject_services/img/mattermost_bot_available_commands.png

doc/project_services/img/mattermost_config_help.png

doc/project_services/img/mattermost_console_integrations.png

...roject_services/img/mattermost_enable_custom_commands.png

doc/project_services/img/mattermost_goto_console.png

...t_services/img/mattermost_slash_command_configuration.png

doc/project_services/img/mattermost_team_integrations.png

doc/project_services/mattermost_slash_commands.md

doc/project_services/project_services.md