Document the output of --show-ssa flag

[LogvinovLeon]

Problem

When trying to reason about performance of Noir programs and comparing - why one version is slower than another - it's useful to understand how does the compiler output as well as the intermediate compiler output differ between two versions.
nargo has the --show-ssa flag that outputs this into, but it's hard to make sense of it without the internal knowledge of compiler implementation details.

This issue - asks for documentation so that we can debug some issues ourselves and better understand what does compiler do under the hood.

Happy Case

The output of --show-ssa is documented and advanced Noir users can read about different compiler passes & the inner workings of compiler front-end.

Project Impact

Nice-to-have

Impact Context

Without it - the only thing we can use to reason about performance is nargo info.
This is not enough as when I write two programs and first takes up 5k gates and second - 6k - there is no way for me to answer the WHY question.

Workaround

None

Workaround Description

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response