[docs] Add SDPLR to list of solvers

[blegat]

I'm already opening this PR even if it's not released to see if the README passes the tests

Basic

• Check that the solver is a registered Julia package, needs Enable ConstraintDual tests SDPLR.jl#19
• Check that the solver supports the long-term support release of Julia
• Check that the solver has a MathOptInterface wrapper
• Check that the tests call MOI.Test.runtests. Some test excludes are
  permissible, but the reason for skipping a particular test should be
  documented.
• Check that the README and/or documentation provides an example of how to
  use the solver with JuMP

Documentation

• Add a new row to the table in docs/src/installation.md

Optional

• Add package metadata to docs/packages.toml

cc @sburer

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (232a95d) 98.28% compared to head (90394bc) 98.28%.
Report is 4 commits behind head on master.

Additional details and impacted files

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##           master    #3644   +/-   ##
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  Coverage   98.28%   98.28%           
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  Files          43       43           
  Lines        5640     5640           
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  Hits         5543     5543           
  Misses         97       97           

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[odow]

I wonder if we should move it to jump-dev before tagging.