docs: Update MariaDB environment variables

[nilanjan-debnath]

add descriptions for UPTIME_KUMA_DB_SSL and UPTIME_KUMA_DB_CA

These documentation updates are required for PR #6671 in the main repo.

[CommanderStorm]

I guess, you don't want this to be documented.. sad, but I am going to accept this as a step towards the ultimate goal 👍🏻

[nilanjan-debnath]

I thought that's all you want me to do.
For those specific changes, simply adding those variables to the .env file should be enough, I suppose.
There are no changes in the docker run command nor the users have to generate any certificate by them self.

Still, if you want me to add something specific, tell me what and where I should add that documentation.
Happy to help