Problem: k8s docs not clear about who they're for

[ttmc]

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.

[muawiakh]

Will merge once travis is 🥗

[codecov-io]

Codecov Report

Merging #2439 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2439   +/-   ##
======================================
  Coverage      88%     88%           
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  Files          40      40           
  Lines        2310    2310           
======================================
  Hits         2033    2033           
  Misses        277     277

[ttmc]

@muawiakh Travis is now 🥗