[EuiFlyout] Improve documentation

[tkajtoch]

Summary

As part of the global flyouts work, we want to improve the documentation of EuiFlyout and related components.

Acceptance criteria

• EuiFlyoutResizable docs should be moved to a separate page and linked from the main EuiFlyout docs page
• Content should be updated to put less focus on prop examples (various sections demonstrating each prop in isolation) and instead provide good prop descriptions in the prop table
• A new section should be added on top of the main EuiFlyout page stating that there are two types of flyouts - regular and resizable (so that when the third type, the child flyout, is ready we can add it there as well)