docs/node-mixin: add fsMointpointSelector to alerts and dashboards

[jan--f]

This adds the option to filter filesystem related alerts and dashboard using the mountpoint label.

[jan--f]

@paulfantom I'd appreciate if you could have a look at this if you have the time.

[raptorsun]

This looks good 👍 Just need some attention from maintainers :)

[simonpasquier]

I'm going to merge this since @discordianfish already approved. Hope this is fine :)