This PR edits the self-managed content for ES + Kibana.

It's huge (mostly due to simultaneous cleanup + downstream link fixes), but I'd like a review on the following pages:

overview

• Self-managed cluster overview

es

• Deploy an ES cluster

install guides, all lightly restructured and reworked into snippets (they were snippets before but I had to redo them)

• Install from archive

• Install from zip

• Install debian

• Install RPM

• JUST ADDITIONAL TOPICS on configure elasticsearch

ignore:

• Important system configuration is largely untouched
• Docker I just split into subpages

note on install docs

imo there's a big gap in these docs around the way you "build" a cluster w/ multiple nodes. I didn't think I could get my head around it and I want to defer it a bit / maybe get some help. I opened an issue here.

kib

• install kib overview
• linux
• windows
• debian
• rpm
• JUST ADDITIONAL TOPICS on configure kib

ignore:

• docker largely untouched
• access kib largely untouched (cute "grant access" section if you're interested)

other

• air gapped: I kept the other sections in here because I was worried they weren't ported over to the other areas properly. largely untouched
• tools and APIs: any additional to add?

A note on snippets

I chose to use snippets because this content is largely repetitive. I think we have an opportunity to merge these pages even further (especially the debian and RPM pages), but I was a little nervous to do that this time given the scope of the content. For reviews, feel free to just write notes with screenshots where you find issues in the installation topics because backtracking through the snippets is hard.

Snippets currently don't cleanly work with headings (sidenav breaks), which is why there are more of them than might seem ideal.