Add Stake pool registration docs

[refi93]

Motivation: Document how to construct a transaction with stake pool registration as an owner, update the docs along the way

Changes:

• Add separate docs where certificates, including stake pool registration are explained in detail
• Update signTransaction docs removing the requirement for at least one tx output
• Simplify notation of paths in doc examples, adding docs for str_to_path() helper function
• Add getSerial() and getVersion() docs