Solution: Add a note atop k8s docs pages to clarify who they're for

Notes:

• This PR mostly adds a "Note" box at the top of several k8s docs pages, to clarify who those docs are for (and not for).
• It also does some minor edits of other text, so the reader doesn't get the impression that this is how we do deployment (in the future).
• I wasn't able to link to the page titled How to Set Up a BigchainDB Network because that page is written in Markdown. I didn't want to link to a specific URL, because it should link to the How to Set Up a BigchainDB Network page in whatever version of the docs that the reader is currently reading.
• I intend to rename the Production Deployment Template page to Kubernetes Deployment Template and do move it later in the contents, but I'll do that in a follow-up PR. I didn't want to do that in this PR because it wouldn't have been as clear what I changed in the text of the k8s docs.