Support in-line LaTeX math output in HTMLWriter

[odow]

index.md

```@example
struct Foo end
function Base.show(io::IO, ::MIME"text/latex", ::Foo)
    return print(io, "\$ x_{1} + x_{2} \$")
end
Foo()
```

make.jl

import Documenter
Documenter.makedocs(
    sitename = "Test",
    format = Documenter.HTML(;
        prettyurls = get(ENV, "CI", nothing) == "true",
    ),
    pages = ["index.md"],
)

Produces

Here's a case from the wild: https://jump.dev/JuMP.jl/stable/tutorials/linear/knapsack/#JuMP-formulation

[odow]

Closed by #2281