Update release docs + release email template

[alamb]

Improve release documentation and add RC to email template subject line as requested in this mailing list thread

[codecov-commenter]

Codecov Report

Merging #450 (009f028) into master (0c00776) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage   82.71%   82.65%   -0.06%     
==========================================
  Files         163      164       +1     
  Lines       44795    45468     +673     
==========================================
+ Hits        37051    37581     +530     
- Misses       7744     7887     +143     

Impacted Files	Coverage Δ
parquet/src/util/test_common/page_util.rs	91.00% <0.00%> (-0.67%)	⬇️
parquet/src/arrow/record_reader.rs	93.44% <0.00%> (-0.54%)	⬇️
parquet/src/column/reader.rs	74.36% <0.00%> (-0.38%)	⬇️
arrow/src/array/transform/mod.rs	86.06% <0.00%> (-0.09%)	⬇️
parquet/src/column/page.rs	98.68% <0.00%> (ø)
parquet/src/schema/types.rs	88.07% <0.00%> (ø)
parquet/src/arrow/arrow_array_reader.rs	78.12% <0.00%> (ø)
parquet/src/file/serialized_reader.rs	94.98% <0.00%> (+0.02%)	⬆️
arrow/src/compute/kernels/filter.rs	91.98% <0.00%> (+0.07%)	⬆️
parquet/src/arrow/array_reader.rs	77.31% <0.00%> (+0.12%)	⬆️
... and 3 more

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