[Discover] Fix document comparison table padding (elastic#205984)

## Summary

This PR fixes the table padding on the Discover document comparison
table which was causing the diff decorations to be cut off. This likely
started after elastic#188495 when some of the standard table styles the
comparison table inherits from changed.

Fixes elastic#205463.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

(cherry picked from commit de3bd71)

src/platform/packages/shared/kbn-unified-data-table/src/components/compare_documents/compare_documents.test.tsx

@@ -109,7 +109,7 @@ describe('CompareDocuments', () => {
         "data-test-subj": "unifiedDataTableCompareDocuments",
         "gridStyle": Object {
           "border": "horizontal",
-          "cellPadding": "s",
+          "cellPadding": "l",
           "fontSize": "s",
           "header": "underline",
           "rowHover": "highlight",

src/platform/packages/shared/kbn-unified-data-table/src/components/compare_documents/compare_documents.tsx

@@ -56,7 +56,11 @@ export interface CompareDocumentsProps {
 
 const COMPARISON_ROW_HEIGHT: EuiDataGridRowHeightsOptions = { defaultHeight: 'auto' };
 const COMPARISON_IN_MEMORY: EuiDataGridInMemory = { level: 'sorting' };
-const COMPARISON_GRID_STYLE: EuiDataGridStyle = { ...DATA_GRID_STYLE_DEFAULT, stripes: undefined };
+const COMPARISON_GRID_STYLE: EuiDataGridStyle = {
+  ...DATA_GRID_STYLE_DEFAULT,
+  cellPadding: 'l',
+  stripes: undefined,
+};
 
 const getStorageKey = (consumer: string, key: string) => `${consumer}:dataGridComparison${key}`;