[Contract examples] Review examples documentation

[claytonneal]

Let's review the contract examples and transaction examples
Specifically, the contract-function-call example which doesn't actually call the contract.
But not only, every improvement is welcome.

Acceptance criteria

• Add a readme in the docs/examples root to explain how the examples are organized
• Add a header to each example ts file, feel free to use AI to autogenerate (but review)
• Have a code snippet for each of the atomic features provided by the SDK (either as runnable examples or in the FAQ)
• Make sure the examples available in docs.vechain.org are injested/generated from the full examples available here