docs: 1.0.0 release notes update: media + feature flags

[garden-of-delete]

SUMMARY

This PR does two things:

1. Responds to feedback from @villebro and others asking for increased quality in the media embedded in the 1.0.0 release notes.
2. Adds a blurb about feature flags to the Summary section of the release notes, and links to a WIP table in the Superset ASF docs documenting the flags and back-end dependencies relevant to the new features in 1.0.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

No test plan needed.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12781 (f6e965e) into master (b32b6cd) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12781      +/-   ##
==========================================
- Coverage   66.93%   66.78%   -0.15%     
==========================================
  Files        1019     1022       +3     
  Lines       49822    50049     +227     
  Branches     4875     4914      +39     
==========================================
+ Hits        33350    33427      +77     
- Misses      16349    16498     +149     
- Partials      123      124       +1     

Flag	Coverage Δ
cypress	50.87% <ø> (-0.06%)	⬇️
javascript	61.68% <ø> (+0.73%)	⬆️
python	63.74% <ø> (-0.42%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.72%)	⬇️
...rontend/src/explore/components/DatasourcePanel.tsx	80.00% <0.00%> (-10.91%)	⬇️
superset/db_engine_specs/hive.py	73.84% <0.00%> (-10.00%)	⬇️
...ntend/src/dashboard/components/PropertiesModal.jsx	75.67% <0.00%> (-7.21%)	⬇️
superset/db_engine_specs/presto.py	81.38% <0.00%> (-7.15%)	⬇️
...ashboard/components/nativeFilters/ColumnSelect.tsx	55.55% <0.00%> (-5.06%)	⬇️
...et-frontend/src/components/Menu/LanguagePicker.tsx	64.28% <0.00%> (-4.95%)	⬇️
...frontend/src/dashboard/containers/FiltersBadge.tsx	89.28% <0.00%> (-3.31%)	⬇️
superset-frontend/src/utils/common.js	70.96% <0.00%> (-3.23%)	⬇️
...nd/src/explore/components/ExploreViewContainer.jsx	77.24% <0.00%> (-2.76%)	⬇️
... and 63 more

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