Add Intake v2 related documentation

[simitt]

Add proper documentation around v2 to APM Server docs and clarify which errors could happen in which Intake API version. Ensure common problem section is valid for both versions or specifies the version.

• Document intake API Changes #1484 Document intake API Changes
• Document data model in ES #1485 Document data model in ES
• Documentation updates: Troubleshooting and tuning #1486 Documentation updates: Troubleshooting and tuning (replaced with Docs: Update Tuning section to show v1/v2 differences #1537)
• Create 6.4 -> 6.5 upgrade guide #1487 Create 6.4 -> 6.5 upgrade guide
• Docs: WIP V2/6.5 additions and changes #1530
• Docs: Update Tuning section to show v1/v2 differences #1537 Update Tuning section to show v1/v2 differences

[watson]

The intake API docs should be updated as well to reflect the new endpoints. The Node.js agent currently links to these and those links need to be updated in the Node.js docs.

[roncohen]

this should include documenting new configuration options introduced for v2