[API-704] v5 SQL: Update documentation

[mdumandag]

Updates the SQL documentation with the changes done in the reference
manual. This change mainly reorganizes the documentation and adds some
notes about the new capabilities. For the details, we directed users
to the reference manual so that we don't have to duplicate most parts
of the reference manual which can change frequently.

[srknzl]

@mdumandag rebase please?

[mdumandag]

@srknzl I did that, but I expect some changes in the Hazelcast reference manual after recent changes (removal of implicit mappings, json -> json-flat, etc.), so I will wait for them. You could review it now, if you want, but note that I might change a couple of things based on the updates to the Hazelcast reference manual

[codecov-commenter]

Codecov Report

Merging #461 (5195975) into master (5c88fdf) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   95.18%   94.81%   -0.37%     
==========================================
  Files         345      345              
  Lines       17633    17633              
==========================================
- Hits        16784    16719      -65     
- Misses        849      914      +65     

Impacted Files	Coverage Δ
...otocol/codec/client_authentication_custom_codec.py	50.00% <0.00%> (-23.81%)	⬇️
hazelcast/reactor.py	81.79% <0.00%> (-8.26%)	⬇️
hazelcast/invocation.py	93.10% <0.00%> (-2.69%)	⬇️
hazelcast/connection.py	91.03% <0.00%> (-2.16%)	⬇️
hazelcast/protocol/client_message.py	93.52% <0.00%> (-1.77%)	⬇️
hazelcast/statistics.py	91.01% <0.00%> (+0.59%)	⬆️

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[srknzl]

Approving but I hope you add a fix for my comment #461 (comment)