docs: Updates to Grafana Installation Documentation

[mildwonkey]

The documentation for manual installation isn't very clear, though I'm not sure what change to suggest. (My very simplest concern is that the "Start Grafana" page is not the most intuitive place to look for instructions on configuring Grafana to start with systemd/init.d).

If you're following the manual installation instructions it isn't clear that you need to return from the downloads page - where there are no instructions beyond unpacking the tarball - and then go on to the Starting Grafana page, because that's where the instructions on configuring Grafana to start with initd or systemd can be found.

The information is all there! It (maybe) could be a bit more obvious to find, or even just more plainly called out from the installation instructions.

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too much extra context:
The impetus for this is that there is information floating around on the internet is no longer accurate, and it was tricky to find the correct info on our website. We had a recent escalation that involved the customer's systemd config - they install everything manually, so when we renamed the grafana binary and they updated, they didn't know to update their systemd configuration. I pulled the info out of our repo, because I couldn't find it on the website until now)

We can see the same problem on an issue from 2015: we renamed the grafana binary, so the setcap command floating around isn't correct. We did it again in G10 and changed the binary name. It's not a huge problem, and user installing via package managers get the updated service files, so this isn't high priority.