[5.x] Update dashboard docs for view/edit mode and landing page (#10965)

[stacey-gammon]

Backports #10965

[stacey-gammon]

We must have a flaky test. I'll have to take a look:

>> FAIL: chrome on any platform - kibana - dashboard app - dashboard tab - should be able to add visualizations to dashboard (49538ms)
Error: tryForTime timeout: NoSuchElement: An element could not be located on the page using the given search parameters.
    at /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/node_modules/leadfoot/lib/findDisplayed.js:37:21
    at /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/node_modules/dojo/_debug/Promise.ts:393:16
    at run (/var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/node_modules/dojo/_debug/Promise.ts:237:8)
    at /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/node_modules/dojo/_debug/nextTick.ts:44:4
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

jenkins, test this