Docs: split `api.md`

[PMeira]

The generated HTML is beyond the default size threshold and running newer Documenter versions result in an error. I'm setting it to ignore that, but we should probably split that file.

This is not an urgent issue; more of an SEO issue than anything.

[PMeira]

One other issue I noticed is that the function signatures are not listed, which can cause confusion for new Julia users.

[tshort]

One option to add method signatures is to use DocStringExtensions.jl.

[PMeira]

One option to add method signatures is to use DocStringExtensions.jl.

That was it! It's already used but it wasn't imported in circuit.jl.

[PMeira]

The generated HTML is beyond the default size threshold and running newer Documenter versions result in an error. I'm setting it to ignore that, but we should probably split that file.

Tested splitting it but some files are still big, like the index, and even the updated docs from Solution. Nothing else useful we can do.

I'll push it with the next changes after DSS C-API is pushed.