[dashboards] Add new Dashboard API docs

[enbashi]

What does this PR do?

Adds documentation and code examples for the new dashboards API.

Motivation

New API.

Preview link

https://docs-staging.datadoghq.com/rami/add_new_dashboard_api/api/#dashboards

Additional Notes

• This new API replaces the timeboards API and soon will also replace screenboards API.
• Python and Ruby code examples are based on the new API which are not released yet. See [dashboards] Add new endpoints & dog commands for new dashboard datadogpy#351 and [dashboards] Add support for the new Dashboard API dogapi-rb#167

[MLaureB]

The docs look good to me!
I have one overall suggestion: I think the Dashboards section should stand right before the Dashboard Lists section (and not at the end).

[MLaureB]

This is an example for the legacy timeboard API: we need to change it with the new one 🙂

[enbashi]

oops. Thanks for catching that.

[l0k0ms]

@MLaureB @enbashi Should we remove the timeboard API doc then since this one replaces it?

[MLaureB]

@l0k0ms , so we should not remove the timeboard API doc completely as we're gonna still have a bunch of people using it who might want to refer to it, but would that be possible to only show the new Dashboards one (and hide the Timeboards one) and put somewhere a link to the legacy timeboards API, in case people still want to access it?

[l0k0ms]

@DataDog/baklava for visibility: this PR now removes the current Timeboard API endpoint doc and replaces it with the new Dashboard endpoint.

It removes the current graphing/dashboard/widget/page and put it into the FAQ section for now. as a freeze.

Next steps are:

1 - Create a new branch to re-introduce the timeboard API as a stand-alone page in the FAQ section in order to keep documentation available in case customers would like to refer to it.

2 - Create a new branch to create a whole widget sub-section in the graphing section that will directly implement the new widget logic, and will be an update on the current widget doc.

[l0k0ms]

PR to introduce timeboard API as a FAQ: #3950 ping @MLaureB

[l0k0ms]

PR to introduce the new widget section: #3948