[Docs] Add a new doc about the node inspection service.

[JoshLind]

Description

This PR makes a few small modifications to the doc site:

1. Rename "Find Nodes" to "Understand Nodes".
2. Add a new "Measure Nodes" section.
3. Move all health check docs to the "Measure Nodes" section.
4. Add a new doc that describes the Node Inspection service and existing node metrics (this was added to the "Measure Nodes" section).

Note: to do this, I had to rearrange a few things. Testing locally seems okay.

Test Plan

Manual verification (i.e., locally deploying and inspecting the new docs format).