Docs: document MM 6.x configuration

[svenseeberg]

I updated to Mattermost 6.x today and changes to the config.json were no longer loaded into the Mattermost server. I had to update the Configurations table to fix the URLs. This PR proposes an update to the bare metal installation.

[Crivaledaz]

Hi,

Thank you very much for your contribution.

For information, Mattermost has added an option to migrate the configuration into the database since Mattermost 5.10 : https://docs.mattermost.com/configure/configuation-in-mattermost-database.html

For the moment, you can use config.json or configuration in database. In the first case, configuration stay the same, but for configuration in database, you are right, changes need to be performed directly on the database via SQL.