Update documentation for v3.2

[khaeru]

Closes #196.
Closes #344.

How to review

Build the documentation and look at the installation instructions and API docs for JDBCBackend.

PR checklist

• Update JDBCBackend documentation to close Document support for in-memory HyperSQL databases #224.
• Update PR and issue templates per message_ix.
• Tests added. N/A
• Documentation added.
• Release notes updated.

[codecov]

Codecov Report

Merging #363 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #363   +/-   ##
=======================================
  Coverage   96.60%   96.60%           
=======================================
  Files          45       45           
  Lines        5243     5243           
=======================================
  Hits         5065     5065           
  Misses        178      178           

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