Update missing future solver interfaces docs

[mrmundt]

Related to #1030

Summary/Motivation:

I realized while staging for more changes that there are some inaccuracies in the docs (specifically, missing highs) and a lack of "how to use APPSI solvers in the legacy SolverFactory".

Changes proposed in this PR:

• Missing highs in future interface docs
• Small extra detail on APPSI page on how to use solvers in the legacy SolverFactory

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