Variety of fixes to the docs

[odow]

• Tidy make.jl
• Enforce strict=true so failing docs not ignored
• Use silent_solver where possible
• Change repl blocks to example

Closes #483, but still running locally so there might be some failures.

[ericphanson]

I think we should add silent_solver=true to the Lasso example: https://jump.dev/Convex.jl/previews/PR484/examples/general_examples/lasso_regression/. It didn't print that much before, not sure why it does now (but maybe it was a bug and Documenter wasn't catching all the output before). Took a brief look at the other examples and they looked fine.

[odow]

@ericphanson any objections to me tagging a v0.15 release?

[ericphanson]

Nope, fine by me! Thanks for all this great maintenance work.