Remove deprecated types, and add note in the docs about this

[mborland]

Closes: #1007

[cppalliance-bot]

An automated preview of the documentation is available at https://1025.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

[codecov]

Codecov Report

❌ Patch coverage is 97.64706% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.5%. Comparing base (2db9ebc) to head (641f87f).
⚠️ Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal128_t.hpp	96.4%	12 Missing ⚠️
include/boost/decimal/decimal32_t.hpp	97.9%	8 Missing ⚠️
include/boost/decimal/decimal64_t.hpp	98.6%	5 Missing ⚠️
include/boost/decimal/decimal_fast128_t.hpp	97.9%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #1025   +/-   ##
=======================================
  Coverage     98.5%   98.5%           
=======================================
  Files          247     247           
  Lines        16459   16459           
  Branches      1857    1857           
=======================================
  Hits         16201   16201           
  Misses         258     258           

Files with missing lines	Coverage Δ
include/boost/decimal/decimal_fast32_t.hpp	95.6% <ø> (ø)
include/boost/decimal/decimal_fast64_t.hpp	97.5% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	98.6% <98.6%> (ø)
include/boost/decimal/decimal_fast128_t.hpp	97.9% <97.9%> (ø)
include/boost/decimal/decimal32_t.hpp	97.9% <97.9%> (ø)
include/boost/decimal/decimal128_t.hpp	96.4% <96.4%> (ø)

... and 1 file with indirect coverage changes

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