docs: add a roadmap page pointing to SIP-53

[mistercrunch]

For if/when SIP-53 passes!

@benceorlai FYI ^^^

[codecov-io]

Codecov Report

Merging #11169 into master will decrease coverage by 0.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11169      +/-   ##
==========================================
- Coverage   65.95%   65.19%   -0.77%     
==========================================
  Files         828      828              
  Lines       39120    39148      +28     
  Branches     3687     3589      -98     
==========================================
- Hits        25801    25521     -280     
- Misses      13212    13516     +304     
- Partials      107      111       +4     

Flag	Coverage Δ
#cypress	55.86% <ø> (-0.18%)	⬇️
#javascript	62.30% <ø> (-0.01%)	⬇️
#python	60.33% <ø> (-1.01%)	⬇️

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Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.72%)	⬇️
superset/examples/unicode_test_data.py	22.00% <0.00%> (-78.00%)	⬇️
superset/db_engine_specs/hive.py	53.90% <0.00%> (-30.08%)	⬇️
superset/db_engine_specs/presto.py	69.76% <0.00%> (-12.32%)	⬇️
superset/examples/world_bank.py	97.10% <0.00%> (-2.90%)	⬇️
superset/examples/birth_names.py	97.36% <0.00%> (-2.64%)	⬇️
superset/examples/helpers.py	95.00% <0.00%> (-2.50%)	⬇️
superset/views/database/mixins.py	80.70% <0.00%> (-1.76%)	⬇️
superset/views/utils.py	82.68% <0.00%> (-1.53%)	⬇️
superset/models/core.py	87.56% <0.00%> (-0.83%)	⬇️
... and 885 more

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