Health Check method

[mDuo13]

• Documents Create health_check rpc rippled#3365
• Groups the "Peer Crawler" and "Validator List" methods along with the new Health Check method into a "Peer Port Methods" category under the rippled APIs heading. Previously the two special methods were stand-alone entries at this level, but three's a crowd so now I've grouped them into a category. The pages' URLs have not changed.
• Clarifies nightly/unstable installs & cleans up some language regarding parallel networks.
• Adds more detail to network bandwidth section of Capacity Planning

The current version of the health check method should be mostly¹ accurate as of v1.6.0-b8. However, I have reported a bug & suggested some changes that I hope may get into the release version. These docs will have to be updated if my suggested changes get applied.

¹ I say "mostly" because the docs don't note the main load_factor bug that causes the method to always report a warning...

[HowardHinnant]

Here's the PR to address the suggested changes: XRPLF/rippled#3491

[lrenazhou1010]

Great doc Rome! Left a few comments/suggestions

[mDuo13]

Broken links include:

• wikipedia Base58 page (recently deleted for "lack of notability"). We'll have to replace this link, but that's outside the scope of this task.
• Some GitHub links that appear to have recovered
• v1.6.0 tag (expected to be broken until the release is ready)