Update and unify stats documentation [JIRA: DOC-227]

[erjohnson]

Add documentation on new stats; ie inspect a node, et cetera.

Can tack on the list of missing stats to this issue @dmitrizagidulin 🍰

[dmitrizagidulin]

We have two major tasks, as far as stats documentation.

1 - Unifying the stat docs
Our stats-related docs are currently scattered between 3 different pages:

• Inspecting a Node - Provides the most complete list of stats, broken down by various section (though the categorization needs to be clarified a little bit, see the discussion of various stat types at riak_status.json docs).
• Statistics and Monitoring - These provide docs for some stats missing in Inspecting a Node. More importantly, this page provides context and categorization, and a discussion of which stats are most useful for graphing and why.
• HTTP Status - Oddly enough, this page lists some stats that are available only through the HTTP GET Stats API call, and not via the CLI. This list should be moved to the main Inspecting a Node page (perhaps in a HTTP-only section).

Proposal: Let's move the full list of stats to the Inspecting a Node page. (Perhaps using https://github.com/basho-labs/riak-help-json to auto-generate a table).
Move the HTTP-only stats from the HTTP Status page to a section in Inspecting a Node.

The Statistics and Monitoring page provides helpful context and a discussion of "minimum useful" set of stats to graph, and should probably remain. (We can use our judgement in terms of duplication of stat tables).

2 - Documenting the un-documented stats

See basho-labs/riak-help-json#1 for a list of ~150 stats that are un-documented on our Basho Docs site.
It would be great to document them (in the unified Inspecting a Node page above, and in the riak_stats.json file).