The node maintenance document should be merged to the main branch

[pruneau628]

@ShakiraKaleel: this document should be merged to the main branch, since this is actually already in place.
https://github.com/CQEN-QDCE/Candy/blob/doc-onboarding/doc/onboarding/maintenance.md.
Mirrors: https://taiga.cqen.ca/project/candy/us/365

[ShakiraKaleel]

I've created a pull request (#64) to add the CandyTest ON nodes' maintenance schedule to the doc/on-boarding branch. Once it is merged, please feel free to create a PR to merge the change to the main branch.