docs: fill in docs page for managing clusters

[jemisonf]

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Related to #5635

Fills in the managing cluster docs, trying to follow the example of the managing applications docs. Happy to adjust style/voice as appropriate. I noticed it looks like the other docs have a max line width that they're using -- I can adjust this to add that but I left it out for the time being since I didn't see a style guide around that.

[jannfis]

Awesome! Thank you for this contribution, @jemisonf! Much appreciated.

LGTM.

[jannfis]

Oh and btw, line length is not critical. I guess it's just me sticking to some 80 characters wrap because of old memories and stuff. But now that you mentioned a style guide, maybe that'd be a very good thing to have!

[jemisonf]

But now that you mentioned a style guide, maybe that'd be a very good thing to have!

It might be nice, even if it's short and not very strict. I don't think it's super high priority but docs do feel a little bit left behind in the contributing section compared to development stuff