[Discover][DocViewer] Adjust bottom margin in flyout (#183478)

## Summary

This PR updates the bottom margin so the JSON view can be scrolled
properly inside DocViewer flyout. It's a follow up for
#183468 (fixes the scrolling to be
limited to flyout height) and
#166406 (taller footer was added
with Close button in it).

src/plugins/unified_doc_viewer/public/components/doc_viewer_source/get_height.test.tsx

@@ -8,6 +8,7 @@
 
 import { monaco } from '@kbn/monaco';
 import { getHeight } from './get_height';
+import { MARGIN_BOTTOM } from './source';
 
 describe('getHeight', () => {
   Object.defineProperty(window, 'innerHeight', { writable: true, configurable: true, value: 500 });
@@ -32,7 +33,7 @@ describe('getHeight', () => {
     const monacoMock = getMonacoMock(500, 0);
 
     const height = getHeight(monacoMock, true);
-    expect(height).toBe(475);
+    expect(height).toBe(500 - MARGIN_BOTTOM);
   });
 
   test('when using document explorer, returning the available height in the flyout has a minimun guarenteed height', () => {

src/plugins/unified_doc_viewer/public/components/doc_viewer_source/source.tsx

@@ -36,7 +36,7 @@ interface SourceViewerProps {
 // inline limitation was necessary to enable virtualized scrolling, which improves performance
 export const MAX_LINES_CLASSIC_TABLE = 500;
 // Displayed margin of the code editor to the window bottom when rendered in the document explorer flyout
-export const MARGIN_BOTTOM = 25;
+export const MARGIN_BOTTOM = 80; // DocViewer flyout has a footer
 // Minimum height for the source content to guarantee minimum space when the flyout is scrollable.
 export const MIN_HEIGHT = 400;