Add How-to guides on parsing Solidity for CLI/Rust/NPM

[Xanewok]

Closes #334

This adds a more hands-on intro to the parsing API and adds two examples on how to use it:

• how to reconstruct the source: it's a kind of a "hello world" since you only visit and print the contents and it helps reinforce that the CST is, in fact, complete (wrt trivia)
• how to list contracts and their names - it's simple, real worldsy and uses two different node kinds in a simple relationship.

This is done for Rust CLI, Rust library and the NPM package used as a library.

With this, we could probably get rid of the content from the respective index pages for Rust/NPM package and use this as the introductory first subpage as it overlaps with it now a bit.

Let me know if this is the direction we want to go in; didn't want to add more involved examples until I was sure that's what we want.

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Latest commit: 3b1b6f6

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[OmarTawfik]

Left a few suggestions.