Add help menu item to header - #29664

Summary

This creates a help menu button in the Kibana header that allows for linking to documentation and/or other types of feedback opportunities.

It currently accepts three props:

1. useDefaultContent: defaults to true and will show the default intro text and link to Kibana's docs site
2. documentationLink: default's to kibana's docs site
3. customContent: adds any custom content below the default content

Additionally, this adds core.chrome.setHelpExtension/getHelpExtension$ and chrome.helpExtension.set/get$ APIs to render a help extension in this popover.

• Use chrome.helpExtension.set() (legacy platform) or core.chrome.setHelpExtension() (new platform) to provide the chrome with a render function that will be called with a DOM element, and should return a function to unmount the rendered component
• Remove the current help extension by passing undefined to chrome.helpExtension.set() or core.chrome.setHelpExtension()
• In an app that uses the Angular router:
  • each route navigation will clear the help extension in the same way that breadcrumbs are cleared
  • angular routes can define a helpExtension on their route and have it automatically rendered like so: https://gist.github.com/spalger/b004091374b426ac1d0323703dae46bc

Example

Shows how to use the default content with additional custom content to show an extra link:

cc @formgeist and ref: #29247

@elastic/kibana-platform team (and specifically @spalger) This is ready to be hooked up. I basically dupped the app menu so it still contains (commented out) that subscription stuff in case that's need to be able to pass down some of those custom options.

Thanks in advance!!!

Also, the default content copy could use some love from @gchaps or @schersh

Checklist

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately