docs: README.md navigation changes for 1.0.0

[garden-of-delete]

SUMMARY

• Changed the "Release Notes" link in the top level README.md to point to a new section at the top of RELEASING/README.md .
• Added a new section to the top of RELEASING/README.md to function as a directory for recent release notes
• Swapped a suboptimal image showing alerts and reports out for two better images

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

No changes to code. No tests required.

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 #12637 (b7dfbdb) into master (7577f47) will decrease coverage by 3.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12637      +/-   ##
==========================================
- Coverage   66.80%   63.25%   -3.56%     
==========================================
  Files        1017      486     -531     
  Lines       49730    29984   -19746     
  Branches     4864        0    -4864     
==========================================
- Hits        33224    18965   -14259     
+ Misses      16383    11019    -5364     
+ Partials      123        0     -123     

Flag	Coverage Δ
cypress	?
javascript	?
python	63.25% <ø> (-0.75%)	⬇️

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

Impacted Files	Coverage Δ
superset/sql_validators/postgres.py	50.00% <0.00%> (-50.00%)	⬇️
superset/views/database/views.py	62.69% <0.00%> (-24.88%)	⬇️
superset/db_engine_specs/sqlite.py	65.62% <0.00%> (-9.38%)	⬇️
superset/db_engine_specs/presto.py	73.37% <0.00%> (-8.66%)	⬇️
superset/databases/commands/create.py	83.67% <0.00%> (-8.17%)	⬇️
superset/databases/commands/update.py	85.71% <0.00%> (-8.17%)	⬇️
superset/sql_validators/base.py	93.33% <0.00%> (-6.67%)	⬇️
superset/db_engine_specs/base.py	79.59% <0.00%> (-6.38%)	⬇️
superset/views/database/forms.py	83.33% <0.00%> (-5.56%)	⬇️
superset/databases/commands/test_connection.py	84.78% <0.00%> (-4.35%)	⬇️
... and 542 more

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